<PAGE>
 
                                                                   EXHIBIT 10.10

                               SUBLEASE AGREEMENT
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THIS SUBLEASE AGREEMENT (this "Sublease") is made and entered into this 22nd of
January, 1998, by and between PSINet, Inc., a New York corporation (the
"Sublessee") and Unisys Corporation, a Delaware corporation, (the "Sublessor").

WHEREAS, by lease dated March 29, 1990 (the "Lease"), Sublessor leased from JBG
Real Estate Associates XVI, Inc., successor to UMT Reston, Inc. (the "Lessor")
certain premises consisting of the Land, Improvements, fixtures and other
interests related to approximately two hundred fifty-seven thousand eight
hundred ninety-two (257,892) rentable square feet contained in two (2) buildings
(the "Buildings") located at 12010 Sunrise Valley Drive, Fairfax County, Reston,
Virginia (collectively, the "Premises"); and

WHEREAS, Sublessor has agreed to sublease a portion of the Premises, consisting
of forty-eight thousand four hundred eighty-two (48,482) rentable square feet on
the 1st Floor ("Space A") in the Phase I building (the "Building") as depicted
on Exhibit "A1" (the "Subleased Premises") to Sublessee on the terms and
conditions hereinafter set forth.

NOW THEREFORE, in consideration of the rents, covenants, agreements,
stipulations and provisions contained herein to be paid, kept and performed by
both Sublessee and Sublessor, the parties do hereby agree as follows:

1.   TERM:  Sublessor does hereby demise and sublease the Subleased Premises to
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     Sublessee and Sublessee does hereby sublease the Subleased Premises from
     Sublessor for a period of eighty-six (86) months commencing on the 1st day
     of February, 1998 (the "Commencement Date") and ending on the 31st day of
     March, 2005 (the "Expiration Date") unless sooner terminated pursuant to
     any provision hereof (the "Term").

2.   USE: The Subleased Premises shall be used for Sublessee's of fice use, data
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     center, computer integration/staging, equipment storage and customer
     presentations, and for no other use whatsoever.

3.   RENT: Sublessee covenants and agrees to pay to Sublessor, without deduction
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     or set off, minimum rental for the Term (the "Base Rent") as set forth on
     Exhibit C attached hereto. In the event any payment of Base Rent or
     Additional Rent (as defined below) is not paid within five (5) business
     days following the date due, Sublessee shall pay to Sublessor, upon demand,
     a late charge equal to five cents for each dollar overdue.

4.   SECURITY DEPOSIT: Concurrently with the execution hereof, Sublessee shall
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     deliver to Sublessor an acceptable unconditional, irrevocable letter of
     credit in the amount of One Hundred Fifty-Three Thousand Five Hundred
     Twenty-Six and 33/ 100 Dollars ($153,526.33) issued by a bank acceptable to
     Sublessor and having an expiration date of no earlier than sixty (60) days
     of following the end of the Term, as the same may be extended (the
     "Security Deposit") to ensure the timely and faithful performance of each
     of the terms and conditions hereof by Sublessee. Sublessor shall hold the
     Security Deposit and shall have the right, but not the obligation, to use
     the same to cure any default by Sublessee hereunder or to repair any damage
     to the Subleased Premises caused by Sublessee, its employees, agents or
     invitees. In the event the original letter of credit expires prior to the
     expiration of the Term, Sublessee shall provide Sublessor with a
     replacement letter of credit at least sixty (60) days prior to its
     expiration. In the 
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     event (i) Sublessee notifies Sublessor at least sixty (60) days prior to
     the expiration of any letter of credit that Sublessee does not intend to
     replace such letter of credit, or (ii) Sublessee does not notify Sublessor
     and does not provide Sublessor with a replacement letter of credit as
     required in this Paragraph 4, Sublessor may, at Sublessor's sole option,
     convert any existing letter of credit into a cash deposit in the full
     amount thereof and Sublessor shall have the right to commingle said funds
     with other funds of Sublessor. In the absence of any default or damage to
     the Subleased Premises, Sublessor shall repay any such cash deposit,
     without interest, to Sublessee within sixty (60) days following the end of
     the Term, as the same may be extended. In the event Sublessor applies the
     Security Deposit or any portion thereof in accordance with this Paragraph
     4, Sublessee shall be obligated immediately to restore the Security Deposit
     to its original amount.

5.   TIME AND PLACE OF PAYMENT: All payments of Base and Additional Rent
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     (collectively, the "Rent") shall be made, in advance, without notice, on
     the first (1st) day of each month during the Term, payable to the order of
     "UNISYS CORPORATION" and addressed to Unisys Corporation, P.O. Box 5585,
     Bismarck, North Dakota 585025585 ATTN: Lease Administration or to such
     other person or at such other place as Sublessor may from time to time
     designate in writing.

6.   OPERATING EXPENSE AND REAL PROPERTY TAX REIMBURSEMENT AND METHOD OF
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     PAYMENT: On and after the Commencement Date, Sublessee shall be obligated
     -------
     to reimburse Sublessor for Sublessee's Proportionate Share of the Building
     or Premises (as defined below) of any rent escalation assessed by Sublessor
     under the terms of this Sublease, including, without limitation,
     escalations with respect to real property taxes and operating expenses.
     Rent escalation amounts and all other sums, except Base Rent, which
     Sublessee is obligated to pay to Sublessor hereunder are collectively
     "Additional Rent". For purposes of calculating Operating Expenses, where an
     operating expense is incurred by Sublessor solely for the Building,
     Operating Expenses hereunder shall be calculated for the Building. Where an
     operating expense is incurred by Sublessor for the Premises, which applies
     to any part of the Premises used in common other than solely attributable
     to the Building, or which applies to all or part of the Premises for which
     the portion allocable to the Building cannot be reasonably determined,
     Operating Expenses shall be calculated for the Premises.

     For purposes of calculating Additional Rent, Sublessor and Sublessee
     acknowledge and agree that "Sublessee's Proportionate Share of the Building
     shall be 44.3%, which percentage is equal to a fraction, the numerator of
     which is forty-eight thousand four hundred eighty-two (48,482) rentable
     square feet in the Subleased Premises, and the denominator of which is one
     hundred nine thousand four hundred seventy-three (109,473) rentable square
     feet in the Building. Sublessor and Sublessee also acknowledge and agree
     that "Sublessee's Proportionate Share of the Premises" shall be 18.8%,
     which percentage is equal to a fraction, the numerator of which is forty-
     eight thousand four hundred eighty-two (48,482) rentable square feet in the
     Subleased Premises, and the denominator of which is two hundred fifty-seven
     thousand eight hundred ninety-two (257,892) rentable square feet in the
     Premises.

     "Additional Rent" shall include Sublessee's Proportionate Share of
     increases in Operating Expenses (as that term is hereinafter defined) for
     each calendar year during the Term of this Sublease above Sublessor's
     Operating Expenses for calendar year 

                                      -2-
<PAGE>
 
     1998. Sublessor and Sublessee acknowledge and agree that Sublessor shall
     charge Sublessee for increases in Operating Expenses based on either
     Sublessee's Proportionate Share of the Building or Sublessee's
     Proportionate Share of the Premises, as appropriate.

     As used herein, the term "Operating Expenses" shall mean the expenses that
     are reasonable, actual, necessary, out-of-pocket, obtained at competitive
     prices paid or incurred by Sublessor during the Term that are directly
     attributable to the operation, maintenance and repair of the Premises which
     are incurred during the calendar year or portion thereof, in accordance
     with sound property management principles and which shall be determined in
     accordance with generally accepted accounting principles consistently
     applied. Operating Expenses based on Sublessee's Proportionate Share of the
     Building or the Premises, as the case may be, shall mean:

     (a)  Wages, salaries and fringe benefits of all employees and third parties
          to the extent actually engaged (though not necessarily physically
          located on the Premises) in the operation, maintenance, administration
          and/or security of the Building or the Premises, including, without
          limitation, employer's share of FICA and any other taxes which may be
          levied on such wages and salaries;

     (b)  All janitorial and office supplies and materials used in the operation
          and maintenance of the Building or the Premises;

     (c)  Cost of all maintenance and service agreements on equipment throughout
          the Building or the Premises, including, without limitation, window
          cleaning, elevator maintenance, fire protection systems maintenance
          and heating, ventilating and air-conditioning maintenance;

     (d)  Taxes for the Building or the Premises;

     (e)  Charges for the usage of gas, common use electricity (HVAC electricity
          and common area electricity) and other fuels and utilities consumed or
          used by Sublessor in the operation and maintenance of the Building or
          the Premises;

     (f)  Expenses of maintenance and repair of exterior areas of the Buildings,
          including, but not limited to, all exteriors of the Buildings, parking
          lot and roadway repair and resurfacing;

     (g)  Expenses of maintenance of common areas of the Building or the
          Premises;

     (h)  Other reasonable expenses incurred and paid to fulfill Sublessor's
          maintenance and repair obligations at the Building or the Premises
          under the Lease; and

     (i)  Insurance premiums for the Building or the Premises.

     Anything to the contrary herein notwithstanding, Operating Expenses shall
     (i) not include (without limitation) submetered electricity pursuant to
     Paragraph 11 herein or any other tenant electric; leasing commissions and
     all other costs, disbursements and expenses incurred for leasing,
     renovating or improving space for tenants; costs for improving, renovating
     or painting tenant space; depreciation or amortization of the Building;
     costs incurred because of tenant violations of a lease; overhead and profit

                                      -3-
<PAGE>
 
     paid to subsidiaries or affiliates of Sublessor for management or other
     services to the extent they exceed competitive costs for such services;
     services and utilities to the extent they are provided exclusively for a
     tenant and the cost is reimbursed by that tenant; interest on debt or
     amortization payments on mortgages, deeds of trust or other debt; repairs
     or work needed because of fire, windstorm or other insurable casualty;
     costs, fines or penalties incurred because Sublessor has violated any
     governmental rule or authority; costs for tests, surveys, cleanup,
     containment, removal, abatement, or other similar activities to remove
     hazardous substances (including, without limitation, asbestos-containing
     materials) from the Premises unless such hazardous substances are in or on
     the Premises because of Sublessee's negligence or willfil misconduct, but
     (ii) shall include costs of a capital nature (including, without
     limitation, capital repairs, capital improvements, capital equipment and
     capital tools) as determined under generally accepted accounting principles
     consistently applied, but only the annual amortization of such costs over
     their respective useful lives with a reasonable salvage value on a
     straight-line basis, and with respect to capital improvements only those
     improvements required by any changes in applicable laws, rules or
     regulations of any governmental authorities enacted after the Commencement
     Date.

     No less than thirty (30) days prior to January 1st of each calendar year,
     Sublessor shall furnish to Sublessee a statement setting forth in
     reasonable detail, the projected Operating Expenses for the subsequent
     twelve (12) month period. Sublessee shall pay to Sublessor in twelve (12)
     monthly installments its share of increases in the projected Operating
     Expenses on the first day of each calendar month during the Term commencing
     January 1, 1999. Within ninety (90) days after the end of each calendar
     year of the Term, Sublessor shall furnish to Sublessee a statement setting
     forth in reasonable detail the actual Operating Expenses for the prior
     calendar year. Within thirty (30) days following the delivery of such
     statement to Sublessee, either (i) Sublessee shall pay to Sublessor its
     share of the amount by which the actual Operating Expenses for said
     calendar year exceeded the projected Operating Expenses for said calendar
     year or (ii) Sublessor shall pay to Sublessee the amount by which
     Sublessee's payments toward increases in the projected Operating Expenses
     for said calendar year exceeded Sublessee's share of increases in the
     actual Operating Expenses for said calendar year.

     Sublessor's failure to timely render a statement with respect to the
     Operating Expenses for any calendar year shall not prejudice Sublessor's
     right to thereafter render a statement with respect thereto (provided that
     Sublessor renders such statement within one (1) year of the subject twelve
     (12) month period) or with respect to any other calendar year. Sublessee
     shall have the right to request supportive documentation from Sublessor
     within ninety (90) days of Sublessee's receipt of the statement from
     Sublessor, and if an error is found, Sublessor shall apply a credit against
     Sublessee's next Rent payment due hereunder.

     Additional Rent for any partial calendar year during the Term shall be
     prorated based upon a 365-day year to reflect the portion of said calendar
     year during which this Sublease was in full force and effect. Sublessee's
     obligation with respect to the payment of Base Rent and Additional Rent due
     for the Term of this Sublease shall survive the expiration or earlier
     termination of this Sublease.

                                      -4-
<PAGE>
 
     As used herein, the Term "Taxes" shall mean all real property taxes, and
     currently due assessments, levies and other charges, general and special,
     ordinary and extraordinary, foreseen and unforeseen, of any kind and nature
     whatsoever (but not including taxes on the net income of Sublessor or
     Lessor), which shall or may be during the Term assessed, levied, charged,
     confirmed or imposed upon or become payable out of or become a lien on the
     Premises and the reasonable expense and advice concerning the potential
     contesting of (to the extent that such contest reduces tax expense) the
     amount, or validity of any such taxes, charges or assessments (including,
     without limitation, attorneys' fees and costs).

     Anything to the contrary herein notwithstanding, "Taxes" shall not include
     income taxes, personal property taxes, franchise taxes, gift taxes,
     transfer taxes, excise taxes, taxes on capital stock, estate taxes,
     successor or inheritance taxes or any penalties, fines or interest for the
     late payment of Taxes.

7.   BUSINESS PRIVILEGE TAXES. BUSINESS USE TAXES AND OCCUPANCY TAXES: Sublessee
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     agrees to pay any revenue tax or charge, occupancy tax, business privilege
     tax, business use tax or any other tax that may be levied against the
     Subleased Premises or Sublessee's use or occupancy thereof during the Term.

8.   RIGHT OF ENTRY: Following the date of this Sublease and prior to the
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     Commencement Date, Sublessee shall have access to the Subleased Premises,
     upon reasonable prior notice to Sublessor, for the purpose of evaluating
     what alterations, repairs or improvements are needed to modify the
     Subleased Premises for its use. Sublessee may not commence any such
     alteration, repair or improvement until Sublessor and Lessor shall have
     consented thereto pursuant to Paragraph 10 and Lessor shall have consented
     to this Sublease. Sublessee's access to the Subleased Premises prior to the
     Commencement Date shall be subject to all of the terms and conditions of
     this Sublease, except for the payment of Rent. Sublessee hereby agrees to
     indemnify and hold harmless Sublessor and Lessor from any and all
     liability, claims, demands, expenses, damages and judgments arising as a
     result of Sublessee's access to the Subleased Premises pursuant to this
     Paragraph 8.

9.   ACCEPTANCE AND SURRENDER OF SUBLEASED PREMISES: Sublessee agrees to accept
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     the Subleased Premises in its present "as is" condition at the date of this
     Sublease, it being both parties intent that Sublessee shall bear the full
     cost and expense of modifying or renovating the Subleased Premises for its
     use. Provided Sublessee is not in default hereunder, Sublessor agrees to
     provide Sublessee with an annual tenant improvement allowance in the amount
     of Twenty-Four Thousand Two Hundred Forty and 96/100 Dollars ($24,240.96)
     during the initial Term which shall be applied as a waiver toward Base
     Rent. Provided Sublessee is not in default under any terms and conditions
     of this Sublease, commencing March 15, 1998 Sublessor agrees to waive
     payment of Two Thousand Twenty and 08/100 Dollars ($2,020.08) of monthly
     Base Rent due in each and every month of the initial Term. If at any time
     during the initial Term Sublessee shall be in default beyond any applicable
     cure period in the observance and performance of any other terms, covenants
     and conditions of this Sublease on Sublessee's part to be observed and
     performed, then the total sum of the Base Rent so conditionally excused by
     operation of the foregoing provision of this Paragraph 9 may at Sublessor's
     sole option become immediately due and payable by Sublessee to Sublessor.
     Until said default is cured, Sublessor shall revoke all future waived
     monthly amounts.

                                      -5-
<PAGE>
 
     Notwithstanding the foregoing, Sublessor shall, at Sublessor's sole cost
     and expense, on or before February 15, 1998, complete the alterations and
     improvements required to demise the Subleased Premises from the remainder
     of the Building (the 4Sublessor's Work"). In addition, Sublessor shall be
     responsible to (i) provide barrier free access in accordance with the
     Americans with Disabilities Act ("ADA.) to all common areas of the Building
     and to two (2) of the four (4) sets of rest rooms in the Subleased
     Premises, and (ii) add fire alarm strobe lights in the Subleased Premises.
     Sublessor will solely bear the cost to submeter electric utility usage to
     the Subleased Premises while Sublessee will solely bear the cost to make
     any other improvements to the Subleased Premises, including any other
     required improvements under the ADA during the Term hereof. Notwithstanding
     the foregoing, Sublessee shall, at Sublessee's sole cost and expense,
     secure all necessary occupancy permits and certificates as may be required
     for Sublessee's occupancy and shall promptly provide copies to Sublessor
     prior to occupancy.

     Sublessor and Sublessee agree to reasonably coordinate Sublessee's
     occupancy of the Subleased Premises so that any permits required to be
     obtained by Sublessor for Sublessor's Work do not delay the Commencement
     Date and Sublessee's occupancy does not violate any jurisdictional code.
     Prior to the Commencement Date, Sublessor shall perform any required
     maintenance on the supplemental free-standing cooling units in the
     Subleased Premises and deliver same in good working order to Sublessee.

     Sublessee shall, at the end of the Term or upon sooner termination of this
     Sublease pursuant to the terms hereof, promptly surrender the Subleased
     Premises in good order and condition and in conformity with the applicable
     provisions of this Sublease and the Lease, excepting only reasonable wear
     and tear.

10.  ALTERATIONS AND MODIFICATIONS: Sublessee agrees to obtain Sublessor's and
     -----------------------------                                            
     Lessor's (if required under the Lease) prior written approval of
     alterations, modifications, repairs or renovations made to the Subleased
     Premises. Sublessor agrees that it shall promptly review plans and drawings
     submitted and that it will not unreasonably delay or deny approval with
     respect to non-structural alterations. Notwithstanding the foregoing,
     Sublessor's consent to such alterations shall be subject to Lessor's
     consent thereto if required. Any alterations, modifications or renovations
     of or to the Subleased Premises shall be limited to partition changes
     (nonbearing walls), electrical, mechanical and plumbing alterations,
     telephone relocations, and decorating. The structural integrity of the
     Building shall not be disturbed in any way. Sublessee shall provide
     Sublessor with partial releases of liens commencing with receipt of the
     second progress payment and a final release of liens upon completion of any
     alterations or modifications to the Subleased Premises executed by all
     contractors or subcontractors who performed such alterations or
     modifications. In addition, Sublessee agrees that all work performed upon
     the Subleased Premises shall be done in a good and workmanlike manner and
     shall be in accordance with all applicable law. All alterations,
     modifications and renovations, upon completion of construction thereof,
     shall become part of the Subleased Premises and the property of Sublessor
     without payment therefore by Sublessor and shall be surrendered to
     Sublessor at the end of the Term or upon sooner termination of this
     Sublease pursuant to the terms hereof; provided, however, that, if
     requested by Sublessor at the time Sublessor grants its approval therefor,
     Sublessee shall, at Sublessee's sole cost and expense, remove all such
     alterations, modifications and renovations, or any part or parts thereof
     specified by Sublessor, from the Subleased Premises and shall repair all
     damage caused by such installation and removal.

                                      -6-
<PAGE>
 
     Notwithstanding anything to the contrary contained in this Paragraph 10,
     Sublessee will secure Sublessor's approval for all work performed by
     Sublessee to the Subleased Premises, including but not limited to,
     design/space planning specifications, architectural and construction
     specifications and contracts, and contractors and subcontractors performing
     work in the Subleased Premises. Sublessee shall be responsible to reimburse
     Sublessor for the reasonable hourly cost of Sublessor's architect to review
     all Sublessee's plans and specifications in conjunction with Sublessor's
     consent thereto. Notwithstanding the foregoing, Sublessor's consent shall
     create no liability or responsibility of any kind on the part of Sublessor
     for the completeness, design sufficiency, or compliance with any laws,
     ordinances, directions, codes, regulations or requirements of governmental
     agencies or authorities as related to Sublessee's drawings, specifications
     or work. Sublessee shall have the responsibility, at Sublessee's sole cost
     and expense, for obtaining all approvals and permits from the governmental
     authorities having jurisdiction over the Subleased Premises for the
     construction of all alterations and related improvements in the Subleased
     Premises. Prior to commencing construction of any alterations, Sublessee
     shall be responsible for (a) obtaining builder's risk and all other
     insurance as required, naming Sublessor and Lessor as additional insureds,
     and (b) indemnifying and holding harmless both Sublessor and Lessor and
     their respective agents, representatives and employees from and against all
     liability, claims, demands, expenses, damages and judgments arising from
     property damage or injury to third parties (including wrongful death)
     during the construction of any alterations unless due to the gross
     negligence or willful misconduct of Sublessor or Lessor or their respective
     agents, representatives or employees. Said insurance and indemnity shall be
     in full force and effect for the period beginning with the commencement of
     construction and ending on the completion of construction. During such
     period, the risk of loss for damage to the alterations occurring because of
     fire or any other casualty relating to Sublessee's work on the Subleased
     Premises shall be absolutely borne by Sublessee, except to the extent
     caused by the gross negligence or willful misconduct of Sublessor, its
     agents, employees or contractors.

     Sublessee shall cause the Subleased Premises and all related improvements
     to be constructed free of any mechanic's or materialmans' lien, claim or
     charge and shall indemnify Sublessor and Lessor absolutely against any and
     all such liability that may arise after the Commencement Date on account of
     the construction, improvements and alterations for which Sublessee is
     responsible. Sublessee shall cause the general contractor and subcontractor
     to file lien waivers and releases as work commences and progresses in
     accordance with procedures and practices in the Commonwealth of Virginia

     Sublessee shall, and shall cause all general contractors and subcontractors
     performing alterations in the Subleased Premises, at all times to use its
     and their reasonable efforts to conduct its and their activities in a
     manner which will not interfere with either Sublessor's use of the Building
     or its business thereon, or Sublessor's use of the Premises or its business
     therein. Sublessee shall be responsible for the removal of debris and
     cleaning of space during the construction of any alterations.

11.  REPAIRS/MAINTENANCE/UTILITIES: With the exception of Sublessee's personal
     -----------------------------                                            
     property and equipment (desks, chairs, systems furniture and telephones) in
     the Subleased Premises and services relating thereto, and subject to
     Sublessee's obligation to pay Base Rent and Additional Rent, Sublessor
     shall at all times, as an 

                                      -7-
<PAGE>
 
     Operating Expense subject to Paragraph 6 hereof, maintain in good order,
     condition and repair the Subleased Premises and every part thereof and all
     fixtures and improvements therein and thereon, through regular inspections
     and servicing, and make replacements to such equipment, systems and
     building components as reasonably necessary throughout the Term, including,
     without limitation, (i) all plumbing and sewage facilities; (ii) all
     windows, doors, entrances and plate glass; (iii) all electrical facilities
     and all equipment including all lighting fixtures, lamps, bulbs and tubes,
     fans, vents, exhaust equipment and systems; (iv) all fire extinguisher
     equipment; (v) the landscaping; (vi) the parking areas; (vii) the exterior,
     floors and roof of the Building; (viii) all HVAC equipment which serves the
     Subleased Premises; and (vix) janitorial services to the Building as
     outlined in Exhibit "D" attached hereto. Sublessor shall also remove all
     debris (including, without limitation, snow and ice) from all sidewalks,
     curbs, parking areas and roadways located upon or adjacent to the Subleased
     Premises. Notwithstanding the foregoing, Sublessee shall be obligated to
     pay Sublessor as Additional Rent for all janitorial services provided in
     Space B, if any.

     Sublessee shall, throughout the Term, at its sole cost and expense, keep
     the Subleased Premises clean, keep waste and drain pipes open and generally
     keep the Subleased Premises and the improvements now or hereafter
     comprising all or any part of the Subleased Premises and the fixtures and
     appurtenances thereto in good order, repair and condition normal wear and
     tear only excepted. In addition, Sublessee shall, at its sole cost and
     expense, promptly repair all damage or injury to the Subleased Premises,
     making replacements, if necessary, caused by (a) the negligence or willful
     misconduct of Sublessee or its employees, agents, invitees, licensees,
     subtenants or contractors; (b) the act of moving in or out of the Subleased
     Premises; and/or (c) the installation and/or removal of any furniture,
     fixtures or other property.

     Electricity to the Subleased Premises for Sublessee's equipment and
     lighting shall be separately metered and usage invoiced monthly to
     Sublessee as Additional Rent, provided, however, that electricity payments
     shall be made in arrears based on metered use. Such usage shall be invoiced
     monthly to Sublessee at the metered rate (without any additional charges
     added thereon) as Additional Rent beginning on the Commencement Date.
     Sublessee shall pay the charges for such utility services directly to
     Sublessor within thirty (30) days of receipt of Sublessor's invoice. In the
     event Sublessee fails to pay its charges for electricity when due,
     Sublessor shall have the right thereafter to require Sublessee to pay to
     Sublessor one-twelfth (1/12th) of the estimated annual electricity costs
     concurrently with the payment of each monthly payment of Base Rent due
     hereunder. In the event Sublessor exercises its right to require the
     payment of electricity charges each month on an estimated basis, promptly
     upon receipt of actual electricity cost figures for the calendar year,
     Sublessor shall, in the case of any overpayment by Sublessee, credit such
     overpayment to the next installment(s) of Base Rent payable hereunder. In
     the case of any underpayments by Sublessee, Sublessee shall pay such sums
     to Sublessor within thirty (30) days of receipt of an invoice from
     Sublessor setting forth the amount due.

12.  LEASE CONTROLLING: Except as herein provided and to the extent any
     -----------------                                                 
     provisions of the Lease apply solely to Lessor and to Sublessor as both
     Lessee and Seller thereunder, Sublessee agrees to comply with all of the
     terms and conditions set forth in the Lease (a copy of which is attached
     hereto as Exhibit "B" and made a part hereof) as are to be performed by
     Sublessor as Lessee thereunder. All of the terms and conditions of the
     Lease shall apply in the same manner to Sublessee as they are expressed
     therein to apply to Sublessor as Lessee thereunder except Sections 2.3, 5,

                                      -8-
<PAGE>
 
     8.3,9.1 (as it relates to Lessee's obligation to rebuild), 16, 37, 42, 43,
     44,45.4, 45.5 and 48 of the Lease and as modified or deleted pursuant to
     the terms of this Sublease. Notwithstanding the foregoing, in the event of
     any conflict between any of the provisions of this Sublease and any of the
     provisions of the Lease, the provisions of this Sublease shall be
     controlling.

13.  LEASE IN EFFECT: Sublessor warrants and represents, to the best of
     ---------------                                                   
     Sublessor's knowledge, information and belief, that the Lease is subsisting
     and is in full force and effect, Sublessor is not in default thereunder,
     and all rents, additional rents and charges due thereunder are and will be
     paid.

14.  SUBLETTING OR ASSIGNMENT: Sublessee covenants that it will not assign its
     ------------------------                                                 
     interest in this Sublease, in whole or in part, or permit the subletting of
     the Subleased Premises or any part thereof without the prior written
     consent of Lessor and Sublessor, which consent of Sublessor shall not be
     unreasonably withheld or delayed.  In the event of any permitted assignment
     or subletting at a rental rate in excess of that being charged to Sublessee
     hereunder, Sublessee shall pay to Sublessor, as collected, fifty percent
     (50%) of such excess rents, less the actual reasonable out-of-pocket costs
     of Sublessee in making such sublease for (a) customary brokerage fees,
     attorneys' fees, and advertising costs, (b) improvement and alteration
     costs to prepare the premises for such sublease, not to exceed prevailing
     market terms, and (c) the amortized cost of any special fixtures or the
     cost of any extra services which Sublessee is providing to such sublessee.
     Notwithstanding the foregoing, Sublessee shall have the right to sublease
     or assign all or a portion of the Subleased Premises to an affiliate or
     subsidiary of Sublessee without Sublessor's consent, provided that
     Sublessee provides Sublessor prior written notice thereof and Sublessee
     furnishes Sublessor with a copy of any such sublease or assignment within
     thirty (30) days of such sublease or assignment.

15.  INSURANCE AND INDEMNITY: Sublessee agrees to indemnify and hold harmless
     -----------------------                                                 
     both Sublessor and Lessor from and against c 11 liability, claims, demands,
     expenses, damages and judgments arising from property damage or injury to
     third parties (including wrongful death) upon the Subleased Premises during
     the Term or any extension thereof, unless due to the gross negligence or
     willful misconduct of Sublessor. Sublessee agrees, at its own cost and
     expense, to keep the Subleased Premises insured under a public liability
     policy against claims for property damage and personal injury to third
     parties (including wrongful death). Such insurance shall be combined single
     limit policy in an amount not less than $3,000,000 per occurrence.  Upon
     execution of this Sublease by Sublessee and at least thirty (30) days prior
     to the expiration date of such policies, Sublessee shall furnish to Lessor
     and Sublessor a certificate or certificates of insurance confirming that
     the required insurance is in full force and effect with all premiums paid
     current. Sublessee further agrees to indemnify and hold harmless Sublessor
     and Lessor from all liability arising out of the filing of any mechanic's
     or materialman's lien against the Subleased Premises by reason of any act
     or omission of Sublessee. Sublessor and Sublessee agree to waive any and
     all right of recovery against the other for each and every insured property
     loss under the terms of any policy or related policies hereto.

16.  PERSONAL PROPERTY: Sublessee agrees to assume full responsibility for its
     -----------------                                                        
     personal property located at the Subleased Premises, and to indemnify and
     hold harmless Sublessor and Lessor against damage sustained by fire, theft
     or other casualty loss.  Sublessee is hereby advised that Sublessor does
     not maintain for the benefit of  

                                      -9-
<PAGE>
 
     Sublessee, nor shall Sublessor at any time hereafter maintain for the
     benefit of Sublessee, any insurance upon Sublessee's personal property,
     fixtures, furnishings and equipment, business interruption, sprinkler
     leakage or glass breakage. Sublessee shall, at its own cost and expense,
     procure all such insurance as Sublessee shall desire for its own benefit.

17.  NOTICES: All notices required shall be given by registered or certified
     -------                                                                
     mail, postage prepaid, return receipt requested. Notice to the Sublessee
     shall be addressed to:

     - PSINet, Inc. 510 Huntmar Park Drive
     - Herndon, VA 20170
     - Attn: Real Estate Department

     Notice to Sublessor shall be addressed to:

     - Unisys Corporation
     - P.O. Box 500
     - Blue Bell, PA 19424
     - ATTN: Real Estate Lease Administration

     All notices shall be deemed received upon receipt or refusal.

18.  HOLD OVER: Notwithstanding any provision of law or any judicial decision to
     ---------                                                                  
     the contrary, no notice shall be required to terminate the Term on the date
     herein specified as the end of the term, and the Term shall expire on the
     date herein mentioned without notice being required from either party. In
     the event that Sublessee remains beyond the expiration date of the Term, it
     is the intention of the parties and it is hereby agreed that a tenancy at
     sufferance shall arise at a monthly rent equal to one hundred fifty percent
     ( 150%) the monthly Minimum Rent in effect at the expiration of the Term
     plus any amounts charged against Sublessor as Lessee under the Lease for
     holdover rent or penalty. It is further agreed that Sublessee shall
     indemnify and hold harmless Sublessor from and against any and all
     liability, claims, demands, expenses, damages and judgments incurred by
     Sublessor as a result of Sublessee's retaining possession.

19.  SUBLESSEE DEFAULT: The occurrence of any one or more of the following
     -----------------
     events shall constitute a default under this Sublease by Sublessee:

     a    The abandonment of the Subleased Premises by Sublessee.

     b.   The failure by Sublessee to make any payment of Minimum Rent,
          Additional Rent or any other payment required to be made by Sublessee
          hereunder on the date due where such failure continues for five (5)
          days after Sublessor's written notice thereof.

     c.   The failure by Sublessee to observe or perform any of the covenants,
          conditions or provisions of this Sublease other than as described in
          the immediately preceding paragraph and/or the failure by Sublessee
          to observe or perform any of the covenants, conditions or provisions
          of the Lease to which Sublessee has agreed to be bound pursuant to
          the terms of this Sublease, where such failure

                                      -10-
<PAGE>
 
          shall continue for a period of fifteen ( 15) days after written notice
          thereof from Sublessor to Sublessee, provided, however, that if the
          nature of Sublessee's default is such that more than fifteen (15) days
          are reasonably required for its cure, then Sublessee shall not be
          deemed to be in default if Sublessee commences such cure with said
          fifteen ( 15) day period and thereafter diligently prosecutes such
          cure.

     d.   The making by Sublessee of any general arrangement or assignment for
          the benefit of creditors; Sublessee becomes a "debtor" as defined in
          11 U.S.C. 101 or any successor statute thereto (unless, in the case of
          a petition filed against Sublessee, the same be dismissed within sixty
          (60) days); the appointment of a trustee or receiver to take
          possession of all or substantially all of Sublessee's assets or of
          Sublessee's interest in this Sublease, where possession is not
          restored to Sublessee within thirty (30) days; or the attachment,
          execution or other judicial seizure of all or substantially all of
          Sublessee's assets or of Sublessee's interest in this Sublease, where
          such seizure is not discharged within thirty (30) days.

     In the event of the failure by Sublessor, after thirty (30) days prior
     written notice thereof, to perform any of the provisions, covenants,
     agreements or conditions of this Sublease on its part to be performed,
     Sublessee may, in addition to any remedies available to it at law or in
     equity, perform the same for and on behalf of Sublessor, the cost of which
     performance, upon the proper payment thereof, shall be paid to Sublessee by
     Sublessor.

20.  REMEDIES: In the event of any such default by Sublessee, Sublessor may at
     --------                                                                 
     any time thereafter, without limiting Sublessor in the exercise of any
     right or remedy which Sublessor may have by reason of such default or
     breach:

     a.   Terminate Sublessee's right to possession of the Subleased Premises by
          any lawful means, in which case this Sublease shall terminate and
          Sublessee shall immediately surrender possession of the Subleased
          Premises to Sublessor. In such event, Sublessor shall be entitled to
          recover from Sublessee all damages permitted to be recovered by a
          landlord pursuant to the laws of the jurisdiction where the Subleased
          Premises are located, together with all damages incurred by Sublessor
          by reason of Sublessee's default, including, but not limited to, the
          cost of recovering possession of the Subleased Premises, reasonable
          attorneys fees, and any real estate commission actually paid.

     b.   Maintain Sublessee's right to possession in which case this Sublease
          shall continue in effect whether or not Sublessee shall have vacated
          or abandoned the Subleased Premises. In such event, Sublessor shall be
          entitled to enforce all of Sublessor's rights and remedies under this
          Sublease, under the laws of the jurisdiction where the Subleased
          Premises are located at law and equity, including the right to recover
          the Minimum Rent, Additional Rent, and all other sums due hereunder as
          the same become due.

     c.   Declare the entire balance of Minimum Rent, Additional Rent and all
          other sums payable hereunder during the remaining Term of this
          Sublease to be immediately due, payable and in arrears as if by the
          terms and provisions of 

                                      -11-
<PAGE>
 
          this Sublease said balance of Minimum Rent, Additional Rent and other
          sums were on that date payable in advance on a present value basis.
          Any such acceleration by Sublessor shall not constitute a waiver of
          any right or remedy of Sublessor.

     d.   Pursue any other remedy now or hereafter available to Sublessor under
          the laws of the jurisdiction where the Subleased Premises are located
          or in equity.

     e.   Pursue any remedy enforceable by Lessor under the Lease.

     To the extent not prohibited by law, all remedies available to Sublessor
     hereunder shall be cumulative and concurrent. No waiver or delay in
     enforcement by Sublessor of any breach of Sublessee's obligations hereunder
     shall constitute a waiver of any such breach or any subsequent breach.

21.  INTEREST: In the event that any sums due and payable to Sublessor pursuant
     --------                                                                  
     to the terms of this Sublease are not paid when due, such sums shall bear
     interest at the rate of twelve percent (12%) per year, from the due date
     until actually paid, unless that rate is usurious as applied to Sublessee
     in which event the rate shall be reduced to the highest non-usurious rate.
     Neither the accrual nor the payment of interest shall cure any default by
     Sublessee under this Sublease.

22.  BROKERS: Sublessor and Sublessee represent, warrant and agree that each has
     -------                                                                    
     not dealt with any broker, agent, finder or other intermediary in
     connection with the subletting of the Subleased Premises except Spaulding
     and Slye Services Limited Partnership, Inc. (the "Listing Broker") and The
     Charles E. Smith Companies (the "Participating Broker"). Sublessor shall be
     solely liable for any commission due to the Listing Broker. The Listing
     Broker shall be solely liable for any commission due to the Participating
     Broker. Sublessor and Sublessee agree to indemnify, defend and hold the
     other harmless from and against any claims against the other resulting from
     a breach or inaccuracy of the foregoing representation, warranty and
     agreement which shall survive expiration, cancellation or other termination
     of this Sublease.

23.  COMPLIANCE WITH LAWS: Except as required of Sublessor herein, Sublessee
     --------------------                                                   
     shall, throughout the Term of this Sublease, observe and comply with all
     statutes, laws, ordinances, notices, orders, rules, regulations and
     requirements of all federal, state and municipal governments and
     appropriate departments, commissions, boards and officers thereof, and
     notices, orders, rules and regulations of the National Board of Fire
     Underwriters, or any other body now or hereafter constituted exercising
     similar functions, foreseen or unforeseen, ordinary as well as
     extraordinary, related to Sublessee's use or manner of use of the Subleased
     Premises, or to fixtures and equipment thereof, and to the extent of any
     alterations or improvements constructed by Sublessee at the Premises at
     Sublessee's sole cost and expense.

24.  AUTHORITY: The parties executing this Sublease represent and warrant that
     ---------                                                                
     they have the full right and lawful authority to execute this Sublease for
     the Term, in the manner and upon the conditions and provisions herein
     contained.

25.  FURTHER DOCUMENTS: Each party agrees to execute and deliver to the other
     -----------------                                                       
     all instruments which may reasonably be required to carry out all terms and
     provisions of this Sublease.

                                      -12-
<PAGE>
 
26.  RECOVERY OF FEES: If either party is successful in enforcing or defending
     ----------------                                                         
     against the other any legal or equitable action or suit for a breach of any
     provision of this Sublease, the successful party shall be entitled to
     recover its expenses and reasonable attorney's fees as determined by the
     court as part of the judgment or decree.

27.  BINDING EFFECT: This Sublease shall be binding upon the successors and
     --------------                                                        
     permitted assigns of Sublessee and Sublessor.

28.  INTEGRATED DOCUMENT: This instrument embodies all of the agreements between
     -------------------                                                        
     the parties with respect to the Subleased Premises, and no oral agreements,
     prior correspondence or other prior writings shall be held to vary the
     provisions hereof. Any subsequent changes or modifications shall become
     effective only by a written instrument duly executed by Sublessee and
     Sublessor.

29.  LESSOR'S CONSENT: This Sublease is contingent upon, and shall have no force
     ----------------                                                           
     or effect until receipt of, the Lessor's written consent hereto.

30.  HOURS OF OPERATION: Sublessor shall provide utility service to the
     ------------------                                                
     Subleased Premises 24-hours a day, seven (7) days a week with the exception
     of annual maintenance shut down and loss of utility service from providers.
     Sublessor shall give Sublessee no less than fourteen (14) days prior
     written notice of annual maintenance shut downs and said shut downs shall
     not continue for more than eight (8) hours. Notwithstanding the foregoing,
     Sublessor will use its reasonable efforts to minimize the duration of such
     annual shut downs. Sublessor shall provide heating, ventilation and air-
     conditioning (HVAC) to the Subleased Premises between the hours of 8:00AM
     and 6:00PM, Monday through Friday and between the hours of 9:00AM and
     1:00PM on Saturday. No HVAC service will be provided on Sundays or
     holidays. Sublessor shall provide Building HVAC to the Subleased Premises
     at other times subject to a charge to be paid by Sublessee to Sublessor at
     the rate of $50.00 per hour as Additional Rent.

31.  COMMON FACILITIES: It is understood by the parties hereto that the
     -----------------                                                 
     cafeteria located at the Premises is operated by a third party franchisee.
     The parties hereto agree that during the Term both Sublessor and Sublessee,
     their employees and invitees, shall have equal access during the times of
     operation as so designated by the respective franchisee. Any fees or costs
     associated with the individual use of this facility shall be borne by the
     individual users of same. Notwithstanding the foregoing, Sublessor shall
     not be obligated to provide a cafeteria at the Premises and Sublessor shall
     not be deemed in default hereunder in the event the cafeteria is
     temporarily or permanently closed.

32.  CASUALTY DAMAGE: If the Subleased Premises are damaged in part or whole
     ---------------                                                        
     from any cause and can be substantially repaired and restored within one
     hundred twenty (120) days from the date of the damage using standard
     working methods and procedures, Sublessor shall, at its expense, promptly
     and diligently repair and restore the Subleased Premises to substantially
     the same condition as existed before the damage. This repair and
     restoration shall be made within said one hundred twenty (120) days unless
     the delay is due to causes beyond Sublessor's control.

     If the Subleased Premises cannot be repaired and restored within said one
     hundred twenty (120) days, then either party may, within then (10) days
     after determining that such repairs and restoration cannot be made within
     the one hundred twenty (120) day 

                                      -13-
<PAGE>
 
     period, cancel this Sublease by written notice to the other party;
     provided, however, that Sublessee shall not be able to cancel this Sublease
     if the damage was caused by Sublessee's negligence or willful misconduct.

     Unless said damage is caused by Sublessee's negligence or willful
     misconduct, the Base Rent and Additional Rent shall abate in proportion to
     that part of the Premises that is unfit for use in Sublessee's business.
     The abatement shall continue from the date of the damage until the earlier
     of ten (10) business days after Sublessor completes repairs and restoration
     and notice to the Sublessee of completion, or until Sublessee again uses
     the Subleased Premises or the part rendered unusable.

33.  CONDEMNATION: In the event that the entire Subleased Premises or the
     ------------                                                        
     portions of the Subleased Premises required for reasonable access to, or
     the reasonable use of the Subleased Premises are taken by eminent domain,
     this Sublease shall automatically end on the earlier of the date of such
     taking or the date when Sublessee is dispossessed. In the event that a
     portion of the Subleased Premises is taken and such taking materially
     interferes with Sublessee's ability to continue its business operations in
     substantially the same manner, Sublessee may end this Sublease on the
     earlier of the date of such taking or the date when Sublessee is
     dispossessed. If there is a partial taking of the Subleased Premises and
     the Sublease continues, the Sublease shall end as to the part taken and the
     Base Rent and Additional Rent shall abate in proportion to the part of the
     Subleased Premises taken and Sublessee's pro rata share shall be equitably
     reduced.  If part or all of the Subleased Premises are condemned for a
     limited period of time not to exceed one hundred twenty (120) days (a
     "Temporary Condemnation"), this Sublease shall remain in effect. The Base
     Rent and Additional Rent and Sublessee's obligations for the part of the
     Subleased Premises taken shall abate during the Temporary Condemnation in
     proportion to the part of the Subleased Premises that Sublessee is unable
     to use in its business operations as a result of the Temporary
     Condemnation.

34.  RULES AND REGULATIONS: Sublessee covenants and agrees that Sublessee, its
     ---------------------                                                    
     employees, agents, invitees, licensees and other visitors shall observe
     faithfully, and comply strictly with, any reasonable rules and regulations
     as Sublessor may, after notice to Sublessee, from time to time adopt;
     provided, however, that in the event of a conflict between the terms of
     such rules and regulations and the terms of this Sublease, the terms of
     this Sublease shall prevail. Sublessee acknowledges that no smoking is
     permitted in the Building (including all common areas and stairwells).
     Sublessee agrees to communicate this "no smoking" rule to its employees,
     agents, invitees, licensees and other visitors, and agrees to enforce this
     rule. Nothing in this Sublease contained shall be construed to impose upon
     Sublessor any duty or obligation to enforce the rules and regulations or
     terms, covenants or conditions in any other sublease as against any other
     sublessee, and Sublessor shall not be liable to Sublessee for violation of
     the same by any other sublessee, its servants, employees, agents, invitees,
     licensees or other visitors.

35.  PERFORMANCE OF SUBLESSEE'S COVENANTS: If Sublessee fails to perform any
     ------------------------------------                                   
     covenant or observe any condition to be performed or observed by Sublessee
     hereunder or acts in violation of any covenant or condition thereof beyond
     any applicable cure period, so long as Sublessee is not proceeding
     diligently to correct or cure such default, Sublessor may, but shall not be
     required to on behalf of Sublessee, 

                                      -14-
<PAGE>
 
     perform such covenant and/or take such steps, including, without
     limitation, entering upon the Subleased Premises, as may be necessary or
     appropriate to meet the requirements of any such covenant or condition,
     provided that Sublessor shall have given Sublessee at least ten (10) days
     prior written notice of Sublessor's intention to do so, unless an emergency
     situation exists, in which case the Sublessor shall have the right to
     proceed immediately; and al costs and expenses incurred by Sublessor in so
     doing, including reasonable legal fees, shall be paid by Sublessee to
     Sublessor upon demand, plus interest at the rate of twelve percent (12%)
     per year from the date of expenditure(s) by Sublessor. Sublessor's
     proceeding under the rights reserved to Sublessor under this Paragraph 35
     shall not in any way prejudice or waive any rights which Sublessor might
     otherwise have against Sublessee by reason of Sublessee's default.

36.  ENVIRONMENTAL: Sublessee shall not dispose of, store, deposit, bury, dump,
     -------------                                                             
     spill, leak, place, release or inject into the Subleased Premises or the
     Premises, any Hazardous Waste (as hereinafter defined) in any manner which
     would violate any of the Environmental Statutes (as hereinafter defined).
     For purposes of this Sublease, the term "Environmental Statutes" shall mean
     all federal, state or local laws, ordinances, rules, regulations or
     policies, now or hereafter existing, which govern or otherwise relate to
     the use, storage, treatment, transportation, manufacture, refinement,
     handling, production or disposal of any Hazardous Waste as defined in the
     Comprehensive Environmental Response, Compensation and Liability Act (42
     U.S.C. S 9, 601 et seq.) ("CERCLA") or any other applicable federal, state
     or local statute. For purposes of this Sublease, the term "Hazardous Waste"
     shall mean any flammable substance, explosive, radio active material,
     hazardous material, hazardous waste, toxic substance, pollutant, pollution
     or any related materials or substances specified in any of the
     Environmental Statutes, including, but not limited to, asbestos, PCB's and
     any hazardous substance as defined in CERCLA.

     Sublessee shall protect, indemnify and save Sublessor harmless from and
     against any and all liability, laws, damage, cost or expense that Sublessor
     may suffer or incur as a result of any claims, demands, damages, losses,
     liabilities, costs, charges, suits, orders, judgments or adjudications
     asserted, assessed, filed or entered against Sublessor by any third party,
     including any governmental authority, arising from the alleged deposit,
     storage, disposal, burial, dumping, injecting, spilling, leaking or other
     use, placement or release in, on or affecting the Subleased Premises or the
     Premises by Sublessee of a Hazardous Waste in violation of any of the
     Environmental Statutes during the Term of this Sublease, including, but not
     limited to, liability for costs and expenses of abatement, correction or
     clean-up, fines, damages, response costs or penalties, or liability for
     personal injury or property damage.

     Sublessor shall protect, indemnify and save Sublessee harmless from and
     against any and all liability, laws, damage, cost or expense that Sublessee
     may suffer or incur as a result of any claims, demands, damages, losses,
     liabilities, costs, charges, suits, orders, judgments or adjudications
     asserted, assessed, filed or entered against Sublessee by any third party,
     including any governmental authority, arising from the alleged deposit,
     storage, disposal, burial, dumping, injecting, spilling, leaking or other
     use, placement or release in, on or affecting the Subleased Premises or the
     Premises of a Hazardous Waste in violation of any of the Environmental
     Statutes by Sublessor, or 

                                      -15-
<PAGE>
 
     by any third party existing prior to the term of this Sublease, including,
     but not limited to, liability for costs and expenses of abatement,
     correction or clean-up, fines, damages, response costs or penalties, or
     liability for personal injury or property damage.

37.  TERMINATION OPTIONS: Provided Sublessee is not in default hereunder at the
     -------------------                                                       
     time such option is exercised and such option commences, Sublessee shall
     have the following options to terminate this Sublease:

     (a)  Sublessee shall have the right to terminate this Sublease in the event
          that Sublessee is unable to connect to fiber optic service, provided
          (i) Sublessee has used commercially reasonable efforts to obtain such
          services, (ii) Sublessor receives written notice from Sublessee
          exercising such option on or before the date which is sixty (60) days
          from the execution of this Sublease by Sublessee and Sublessor, and
          (iii) Sublessee accompanies its notice with a payment to Sublessor
          comprised of Ninety-One Thousand Nine Hundred Seventy-Three and 67/
          100 Dollars ($91,973.67) plus an amount equal to Sublessor's
          transaction costs to consummate this Sublease, as reasonably
          determined by Sublessor, including, but not limited to, demising and
          electrical submetering costs and architectural or engineering costs
          related thereto, legal fees, cost of obtaining Lessor's consent and
          any brokers' commissions related to Paragraph 22 of this Sublease to
          the extent they have been paid; and

     (b)  Sublessee shall have the right to terminate this Sublease effective
          September 30, 2003, provided (i) Sublessor receives written notice
          thereof from Sublessee on or before December 31, 2002, and (ii)
          Sublessee accompanies its notice with a payment to Sublessor comprised
          of Three Hundred Forty Thousand Thirty-Three and 76/100 Dollars
          ($340,033.76) plus an amount equal to Sublessor's unamortized
          "transaction costs" amortized over the Sublease Term at an interest
          rate of ten percent (10%).

38.  RIGHT OF FIRST OFFER: Provided that Sublessee is not in default at the time
     --------------------                                                       
     such option is exercised and such option commences, Sublessee shall have
     the right of first offer for leasable premises in the Buildings during the
     Sublease Term. If all or any portion of the Buildings becomes available to
     lease, Sublessor shall first offer, in writing, to lease such space to
     Sublessee. The location of the space shall be determined by Sublessor in
     its sole but reasonable discretion. Such offer to Sublessee shall be upon
     terms and conditions as shall be mutually agreed to by the parties at the
     time such space becomes available. Sublessee shall have thirty (30) days
     following receipt of Sublessor's notice in which to negotiate mutually
     acceptable terms and to elect, in writing, to lease such space on such
     mutually acceptable terms from Sublessor.  

     In the event Sublessee shall fail to exercise such option with such thirty
     (30) day period, Sublessor hereafter may lease such space to any third
     party without any additional notice to Sublessee at a rent and upon such
     terms and conditions as Sublessor may determine and Sublessee shall have no
     further rights under this Paragraph 38 to such space or any future space.
     Notwithstanding the foregoing, if Sublessor shall desire to lease such
     space to a third party at terms at least ten percent (10%) less than those
     offered to Sublessee under this Paragraph 38, Sublessor shall re-offer such
     space to Sublessee at such lesser terms and Sublessee shall have five (5)
     days in which to exercise such second offer.

                                      -16-
<PAGE>
 
     In the event Sublessee exercises any of the rights set forth herein,
     Sublessor and Sublessee shall execute an appropriate amendment to this
     Sublease. In the event Sublessee leases all of the Building in its entirety
     from Sublessor, Sublessee's termination option set forth in Paragraph 37
     (b) hereto shall become null and void and of no further force and effect.

39.  OPTION TO RENEW: Provided Sublessee is not in default hereunder and
     ---------------                                                    
     Sublessor exercises Sublessor's option to renew the Lease, Sublessee shall
     have the option to renew this Sublease for Sublessor's renewal term upon
     all of the terms and conditions of this Sublease as then in effect, except
     that Sublessee's Base Rent and base year for purposes of Operating Expenses
     shall be adjusted, provided that Sublessee's Base Rent during the renewal
     term shall not exceed ten percent ( 10%) above Sublessor's costs during the
     initial year of the renewal term. In the event Sublessee desires to renew
     this Sublease, it shall give written notice to Sublessor of its intention
     to renew this Sublease at least twelve (12) months prior to the Expiration
     Date. Sublessor shall provide Sublessee with written notice of its election
     to renew the Lease simultaneous with Sublessor's notice to Lessor.

40.  SIGNS: (a) Subject to Sublessor's receipt of all necessary approvals and
     -----                                                                   
     permits, Sublessee shall have the non-exclusive right to have Sublessor
     install, at Sublessee' s sole expense, Sublessee's corporate name and/or
     logo on a monument sign on or adjacent to Sublessor's existing sign located
     at the main Sunrise Valley Drive entrance to the Premises. Subject to the
     Lease, the location and size of said monument signage shall be within
     Sublessor's sole discretion and Sublessor shall diligently pursue Lessor's
     consent to such monument sign pursuant to this Paragraph 40(a).

     (b) Subject to Sublessee's receipt of all necessary approvals and permits,
     Sublessee shall have the exclusive right to erect, at Sublessee's sole
     expense, a monument sign near the South entrance to the Building and to
     install, at Sublessee's sole expense, its corporate logo on the glass
     entrance doors to the South lobby of the Building, pursuant to the site
     plan, detail drawings and specifications as depicted on Exhibit "E"
     attached hereto.

41.  PARKING: Sublessor shall provide unreserved parking, free of charge, at a
     -------                                                                  
     ratio of three (3) spaces per 1,000 rentable square feet on the adjacent
     surface lots to the Building. Included in this ratio shall be five (5)
     spaces proximate to the South lobby of the Building, which shall be
     designated for Sublessee's visitors.

42.  GENERATORS: Sublessee shall have the right to install up to three (3)
     ----------                                                           
     generators and associated equipment at the Premises on the Land as provided
     on Exhibit "F" attached hereto subject to Sublessee's submission to
     Sublessor and Lessor of detailed plans and specifications for Sublessor's
     and Lessor's consent. In the event Sublessee proposes to have the
     generators located on the roof of the lower level space and the  roof needs
     structural modifications to accommodate such generators, said structural
     modifications shall be done at Sublessee's sole cost and expense. Sublessee
     shall also have the right, subject to Sublessor's consent, which shall not
     be unreasonably withheld or delayed and any Lessor consent as may be
     required, to place for no more than one hundred eighty (180) days from the
     Commencement Date a thirty (30) foot aluminum trailer beside the lower
     level space and wire the generator contained in the trailer into the
     Subleased Premises. Notwithstanding the foregoing, in the event the 

                                      -17-
<PAGE>
 
     trailer is not removed within such one hundred eighty (180) days, Sublessor
     shall have the right to charge Sublessee One Thousand and No/ 100 Dollars
     ($1,000.00) per day as Additional Rent for every day the trailer remains at
     the Premises, and Sublessee shall indemnify Sublessor and give Sublessor
     the right to remove the trailer at Sublessee's sole cost and expense.

43.  SECURITY: Sublessor shall provide Sublessee with sensor card key access to
     --------                                                                  
     the Building with three (3) sensor cards per one thousand (1,000) rentable
     square feet leased at no charge. Sublessor and Sublessee shall reasonably
     coordinate for the Building security staff to (a) use the rest rooms
     located in the Subleased Premises; (b) walk daily through the Subleased
     Premises (non-disclosure required); (c) provide periodic ingress and egress
     reports as reasonably requested by Sublessee; and (d) deactivate terminated
     employees' access cards upon Sublessee's written request.

44.  TRENCHING RIGHTS: Sublessee shall have the right to lay conduit on the Land
     ----------------                                                           
     subject to Sublessee's submission to Sublessor and Lessor (if required by
     the Lease) of detailed plans and specifications of Sublessee's proposed
     installation of trenching for conduit for Sublessor's approval which shall
     not be unreasonably withheld or delayed and for Lessor's approval if
     required under the Lease.

45.  ROOF RIGHTS: (a) Sublessee shall have the right, pursuant to any conditions
     -----------                                                                
     of limitation as provided herein or pursuant to law, to furnish, install,
     maintain and operate on the roof of the Building (hereinafter referred to
     as the "Roof") two (2) satellite dishes with a screened enclosure if
     requested by Sublessor (referred to as the "Antennae:") including
     appurtenant lines to any offices located within the Building, and provided
     that the furnishing, installation, maintenance and operation of the
     Antennae and the electrical power supply and use thereof shall be subject,
     in all respects, to all the covenants, agreements, terms, provisions and
     conditions contained in this Sublease. Sublessee understands and agrees
     that access to the Roof is obtained only through the assistance of
     Sublessee's designated personnel and, if desired during other than normal
     business hours, Sublessee shall pay to Sublessor a reasonable charge for
     the additional services required thereby. Sublessee understands and agrees
     to accept such portion of the Roof, or the Building in "as-is" condition.

     (b) Sublessee, at Sublessee's cost and expense and at the sole Ask of
     Sublessee, shall, after the Commencement Date, if and to the extent
     permitted by law and subject to Sublessor's and Lessor's approval, as
     required herein, furnish, install, maintain and operate the Antennae on the
     Roof and provided, however, that the Antennae and each and every part and
     component thereof (including, but not limited to, all electrical
     connections and outlets and lightening arresters and conductors) shall be
     furnished and installed in accordance with plans as approved by a certified
     professional engineer licensed by the Commonwealth of Virginia and by
     Sublessor and Lessor, and maintained and operated in accordance with sound
     engineering principles and in compliance with all rules, orders,
     regulations, requirements and laws of all governmental authorities
     applicable to the Premises and to Sublessee's installations and operations
     thereon and therefrom. The furnishing and installation of the Antennae by
     Sublessee shall include such safeguards as may be necessary for the
     servicing of same, including, if deemed reasonably necessary by Sublessor,
     the construction of a platform and handrails. 

                                      -18-
<PAGE>
 
     (c) Prior to the furnishing, installation and maintenance of the Antennae,
     Sublessee shall submit plans and specifications relating to any such
     furnishing, installation and maintenance and shall request Sublessor's and
     Lessor's approval therefor, and no such furnishing, installation and/or
     maintenance shall be undertaken, started or begun by Sublessee unless and
     until Sublessee has received the prior written approval from Sublessor and
     Lessor, including, but not limited to, approval as to the location of the
     portion of the Roof where the Antennae are to be so installed, it being
     expressly understood and agreed by Sublessee that any such approval
     required from Sublessor may be withheld if in Sublessor's judgment the
     installation or operation of the Antennae would adversely affect the use,
     enjoyment or occupancy by any tenant or occupant of the Building or would
     adversely affect the operation of any Building equipment; Sublessor will
     not accept any responsibility for the plans as a result of its review and
     approval.

     (d) Sublessee, at Sublessee's expense, shall keep the Antennae in good
     order and condition and Sublessee shall, at Sublessee's expense, make all
     repairs and replacements thereto and to the Building, ordinary or
     extraordinary, structural or otherwise, foreseen or unforeseen, when
     necessary by reason of the furnishing, installation, maintenance or
     operation of the Antennae or any replacements, and Sublessee will comply
     with all applicable rules, laws and regulations and will bear all expenses
     of complying with them.

     (e) Sublessee, at Sublessee's expense, shall on or before the expiration or
     other termination of the term of this Sublease, remove the Antennae and any
     replacements thereof and shall repair all damage and injury to the
     Building, including the Roof, caused by the removal of the Antennae or any
     replacements thereof and shall restore the Building, including the Roof, to
     the condition existing prior to the installation of the Antennae.

     (f) Sublessee, at Sublessor's expense, shall, upon written request of
     Sublessor, within forty-five (45) days, relocate the Antennae to such other
     useable location on the Premises in any such case, as may be designated by
     Sublessor if Sublessor deems any such relocation necessary or desirable for
     the efficient operation or maintenance of the Building or any Building
     equipment.

     (g) Sublessee covenants and agrees that at no time (i) will the Antennae be
     used or operated in any manner so as to interfere with any operation of the
     Building or of any tenant, occupant or licensee of the Building, and (ii)
     will the Antennae interfere in any way with the functioning of any
     electronic gear, mechanism, computer or system utilized in the Building
     such as, but not limited to, the electronic elevator control system.

     (h) Nothing contained herein shall be deemed or construed to constitute a
     representation or guarantee by Sublessor that the Antennae may, as a matter
     of law, be so installed, maintained or operated by Sublessee.

     (i) Sublessee, recognizing that the Premises has been developed and is
     being maintained as a location for an outstanding type of business
     occupancy, and as a special inducement to Sublessor to grant this right,
     covenants and agrees that at all times the appearance, quality and utility
     of all equipment used by Sublessor under 

                                      -19-
<PAGE>
 
     this Paragraph 45 will be only such as meets with Sublessor's approval and
     if at any time reasonably disapproved by Sublessor, Sublessee shall remove
     the reasonable basis for such disapproval in such manner and within such
     reasonable time as may be specified by Sublessor in a written notice given
     by it to Sublessee for such purpose.

     (j) Sublessee agrees that it will not during installation, maintenance or
     removal of the Antennae, either directly or indirectly, use any contractors
     and/or labor and/or materials if the use of such contractors and/or labor
     and/or materials would or will create any difficulty with other contractors
     and/or labor engaged by Sublessor or others in the construction,
     maintenance and/or operation of the Premises or any part thereof. Sublessee
     represents that the Antennae will not require any structural modifications
     to the Building. In the event structural modifications are necessary,
     Sublessee shall be responsible for same. Sublessee agrees to utilize a non-
     penetrating roof mounting method for the Antennae and to use the existing
     pitch pockets on the Roof for the installation of cabling from the
     Antennae. In the event that use of any existing pitch pocket is not
     technically feasible, Sublessee shall use Sublessor's designated roofing
     contractor to make al roof penetrations. In the event that a roof
     penetration is made pursuant to the foregoing and Sublessee discovers that
     the roofing materials contain asbestos, Sublessor agrees to reimburse
     Sublessee for the additional cost incurred by Sublessee, if any, to install
     the Antennae on the Roof of the Building resulting from the presence of
     asbestos requiring special handling under applicable law.

     (k) Sublessee shall, and Sublessee covenants and agrees to, indemnify and
     save harmless Sublessor against and from any and all claims by or on behalf
     of any person or persons, firm or firms, corporation or corporations
     arising out of and/or in connection with Sublessee's installation and
     operation of the Antennae and activities related thereto in and on the
     Building and arising from or in connection with the use of or from any work
     or thing whatsoever done (other than by Sublessor or its agents or
     employees) in or on the Building or any part thereof by Sublessee related
     to the Antennae during the term of this Sublease, and further indemnify and
     save Sublessor harmless against and from any and all claims, liabilities
     and penalties imposed by any governmental authority by reason of the
     installation or operation of the Antennae or arising from any condition or
     use of, or any work or thing whatsoever done in or on, the Building or any
     part thereof due to or arising from any default of Sublessee in the
     performance of any of its obligations under this paragraph or from any act
     or negligence of Sublessee or any of its agents or employees, and from and
     against all costs, expenses and liabilities incurred in or in connection
     with any such claim or claims or action or proceeding brought thereon.

     (l) If any governmental license or permit shall be required for the proper
     and lawful operation of Sublessee's Antennae, Sublessee, at Sublessee's
     expense, shall duly procure and thereafter maintain such license or permit
     and submit the same to inspection by Sublessor. Sublessee, at Sublessee's
     expense, shall at all times comply with the terms and conditions of each
     such license or permit.

     (m) Sublessee covenants and agrees that neither its rights under this
     paragraph nor any interest therein shall be assigned, mortgaged, pledged,
     encumbered or otherwise transferred (whether voluntarily, by operation of
     law or otherwise) without the prior written consent to Sublessor.
     Notwithstanding anything else to the contrary, nothing herein shall operate
     to affect Sublessee's interest hereunder in the event of any merger or
     consolidation of Sublessee.

                                      -20-
<PAGE>
 
     (n) Whenever at any time during the term of this Sublease a charge
     adjustment for additional electrical current required for operation of the
     Antennae shall be incurred by Sublessor, Sublessor shall furnish to
     Sublessee a statement in writing of Sublessor's determination of the
     appropriate amount of said adjustment in said monthly charge payable by
     Sublessee, said statement to include sufficient detail to enable Sublessee
     to verify Sublessor's determination of the amount of the adjustment for
     inspection by Sublessee, such cost and other records of Sublessor as were
     used by it as the basis for its computation.

     (o) In the event that Sublessee shall require additional electrical energy
     in excess of such reasonable quantity to be furnished as herein provided
     and if, in Sublessor's judgment, such excess requirements can not be
     furnished unless additional risers, conduits, feeders, switchboards and/or
     appurtenances are installed in the Building, mechanical penthouse or on the
     Roof, Sublessor, upon written request of Sublessee, will proceed with
     reasonable diligence to install such additional risers, conduits, feeders,
     switchboards and/or appurtenances provided the same and the use thereof
     shall be permitted by applicable laws and insurance regulations and shall
     not cause permanent damage or injury to the Building, mechanical penthouse,
     or Roof, or cause or create a dangerous or hazardous condition or entail
     excessive or unreasonable alterations or repairs or interfere with or
     disturb other tenants or occupants of the Building, mechanical penthouse or
     Roof, and Sublessee agrees to pay all costs and expenses incurred by
     Sublessor in connector with such installation.

     (p) In order that Sublessor may at all times have all necessary information
     which it requires in order to maintain and protect its equipment, Sublessee
     agrees that Sublessee will not make any alterations to the electrical
     equipment and/or appliances in or on the Building or Roof related to the
     Antennae except as permitted hereunder.

     (q) The failure of Sublessor to insist in any one or more instances upon
     the strict performance of any one of the covenants, agreements, terms,
     provisions or conditions of this paragraph or to exercise any election
     herein contained shall not be construed as a waiver or relinquishment for
     the future of such covenant, agreement, term, provision, condition or
     elector, but the same shall continue and remain in full force and effect.
     No waiver by Sublessor of any covenant, agreement, term, provision or
     condition of this paragraph shall be deemed to have been made unless
     expressed in writing and signed by Sublessor.

46.  MISCELLANEOUS: (a) Sublessor shall have two (2) business days following
     -------------                                                          
     Sublessee's execution of this Sublease to countersign the Sublease and
     forward it to Lessor for Lessor's consent. In the event Sublessor's
     execution is delayed, the Commencement Date and Base Rent commencement date
     shall be delayed one (1) day for each day beyond the second (2nd) business
     day which the Sublease is not executed by Sublessor. Sublessor shall use
     its reasonable efforts to obtain Lessor's consent to this Sublease within
     seventeen (17) days of Sublessee's execution thereof, and (b) any
     capitalized terms used herein and not otherwise defined shall have the same
     meaning as defined in the Lease.

                                      -21-
<PAGE>
 
IN WITNESS WHEREOF, the parties hereto have executed this Sublease Agreement as
of the day any year first above written.


WITNESS:                                SUBLESSEE:
                                        PSINET, INC.



By: /s/ Cheri L. Clark                   By: /s/ Harold S. Wills              
    -------------------------------          -----------------------------  
                                        Title: Chief Operating Officer
                                               ---------------------------



WITNESS:                                SUBLESSOR:
                                        UNISYS CORPORATION



By: /s/ M. Kurnel                        By: /s/ Gregory T. Fischer 
   -----------------------                   ----------------------------- 
                                                  Gregory T. Fischer 
                                                  Vice President
                                                  Facilities & Asset Management

                                      -22-
<PAGE>
 
February 17, 1998

VIA FEDERAL EXPRESS
-------------------

Ms. Diana J. Sperle
Unisys Corporation
Township Line & Union Meeting Roads
P.O. Box 500
Blue Bell, Pennsylvania 19424-0001

Re:  Proposed Sublease by and between Unisys Corporation ("Tenant") and PSINet,
     Inc. ("Subtenant")

Dear Ms. Sperle:

     In accordance with the terms of Section 12 of the Lease dated as of March
29, 1990 (the "Master Lease") by and between Tenant and TrizecHahn Reston Unisys
I LLC and TrizecHahn Reston II LLC ("Landlord"), as successor in interest to UMT
Reston, Inc., Landlord is willing to consent to the proposed Sublease by and
between Tenant and Subtenant, a copy of which is attached hereto as Exhibit "A"
and made a part hereof (hereinafter the "Proposed Sublease"), upon the terms and
conditions set forth below, NOTWITHSTANDING ANYTHING TO THE CONTRARY IN THE
PROPOSED SUBLEASE. The Proposed Sublease as modified by this letter is
hereinafter referred to as the "Sublease".

     1.   Subtenant shall indemnify Landlord and its agents against all claims,
damages, costs and expenses arising out of Subtenant's failure to perform or
observe any of the terms and conditions of the Master Lease which relate to the
premises which are the subject of the Sublease (hereinafter the "Subleased
Premises"), and further agrees that if Subtenant breaches any of such terms and
conditions, Landlord shall have all remedies against Subtenant available to
Tenant thereunder, except as hereinafter provided. The Sublease is in all
respects subject and subordinate to the Master Lease. The last sentence of
Paragraph 12 of the Proposed Sublease is hereby deleted and the following
language is inserted in lieu thereof:

          "Notwithstanding the foregoing, in the event of any
          conflict between any of the provisions of this Sublease
          and any of the provisions of the Lease, (i) as between
          Sublessee and Sublessor, the provisions of this
          Sublease shall be controlling, and (ii) as between
          Sublessee and Lessor, to the extent that any provision
          of this Sublease imposes an obligation on Lessor that
          does not exist in the Master Lease or makes an
          obligation of Lessor that exists in the Master Lease
          more burdensome to Lessor than it would
<PAGE>
 
Ms. Diana J. Sperle
Unisys Corporation
February 17, 1998
Page 2

          have been in the absence of the Sublease, then, in
          either of such events, the provision of the Lease shall
          be controlling."

     2.   Tenant and Subtenant shall provide written notice to Landlord of (i)
any extension of the Sublease in accordance with the terms of Paragraph 39
thereof prior to the commencement of any such additional term, and (ii) any
acceptance by Subtenant of an offer of additional space by Tenant to Subtenant
pursuant to Paragraph 38 thereof.

     3.   During the term of the Sublease, Landlord shall send to Subtenant
copies of any and all notices of default of any kind or nature which are sent to
Tenant, simultaneously with the transmittal of such notices to Tenant, and
Landlord shall accept a full cure by Subtenant of any such default during the
period provided in the Master Lease for the cure thereof as if it had been fully
cured by Tenant during such period.

     4.   Subtenant shall obtain and at all times during the term of the
Sublease maintain, at its sole cost and expense, policies of insurance covering
its fixtures, property and equipment installed and located in the Subleased
Premises, in such amounts as are set forth in the Master Lease; provided,
however, that Section 8.3 of the Master Lease shall not apply to Subtenant.
Subtenant shall provide and keep in force during the term of the Sublease with a
company or companies approved by the Landlord a comprehensive general liability
insurance policy as set forth in the Proposed Sublease. All such policies of
insurance shall name Tenant, Landlord and Landlord's management agent as
additional insureds.

     5.   Subtenant agrees that it will not assign or encumber, or permit to be
encumbered, its right or interests under the Sublease, nor sublet the whole or
any part of the Subleased Premises, without the prior written consent of
Landlord, which consent shall not be unreasonably withheld, conditioned or
delayed.

     6.   No provision of the Sublease may be amended except in a writing signed
by Tenant and Subtenant and consented to by Landlord, which consent shall not be
unreasonably denied, conditioned or delayed; provided, however, that so long as
an amendment does not, and would not have the effect of, increasing any of
Landlord's obligations or abridging any of Landlord's rights under the Master
Lease, Landlord's consent shall not be required with respect to any amendment of
(a) Paragraph 1 of the Proposed Sublease if the same does not have the effect of
causing the term of the Proposed Sublease to extend beyond the expiration of the
term of the Master Lease, or (b) Paragraphs 3, 4, 5, 6, 7, 8, 9, 11, 13, 16, 17,
18, 20, 21, 22, 24, 26, 27, 28, 30, 31, 32, 33, 34, 35, 37 and 43 of the
Proposed Sublease.
<PAGE>
 
Ms. Diana J. Sperle
Unisys Corporation
February 17, 1998
Page 3

     7.   Neither the Master Lease nor the Sublease shall be deemed to grant
Subtenant any rights whatsoever against Landlord. Subtenant acknowledges and
agrees that its sole remedy for any alleged or actual breach of its rights in
connection with the Sublease shall be solely against Tenant.

     8.   The Sublease shall not release Tenant from any existing or future
duty, obligation or liability to Landlord pursuant to the Master Lease, nor
shall the Sublease change, modify or amend the Master Lease in any manner. In
particular, the Sublease shall not absolve (a) Tenant from the requirements set
forth (i) in Section 7.5 of the Master Lease that Tenant obtain Landlord's prior
written approval, to the extent Landlord's prior written approval is required by
the terms and conditions of said Section 7.5, for alterations, improvements and
additions to the building(s) or the land on which the Subleased Premises is
situated ("Alterations"), as applicable, including, but not limited to, those
contemplated by Paragraphs 9, 10, 11, 42, 44 and 45 and Exhibit F to the
Proposed Sublease, and that any such Alterations comply with the requirements
set forth in said Section 7.5, and (ii) in Section 12 of the Master Lease that
Tenant obtain Landlord's prior written approval for any further subleases, (b)
Tenant from reimbursing Landlord for all of Landlord's expenditures made in
connection with any Alterations, pursuant to Section 7.5 (b) of the Master
Lease, or (c) Subtenant from the obligation to vacate the Subleased Premises in
the event that Landlord exercises any of its termination rights under Section
9.3 and from the requirements set forth in Sections 14, 25, 28, 29 and 46 of the
Master Lease. Further, Tenant and Subtenant acknowledge and agree that the
granting of Landlord's consent to any of the work referred to in clause (a)(i)
above shall not be deemed a representation or warranty by Landlord of the
sufficiency of the plans therefor, the quality or character of any such work,
its compliance with all applicable building codes, or otherwise. Landlord agrees
that Subtenant shall have no liability or obligation to Landlord under Section
45 of the Master Lease, in consideration for which Subtenant agrees that its
indemnification obligation under Section 36 of the Sublease shall also include
an indemnification of Landlord by Subtenant, upon the same terms and conditions
as are set forth in said Section 36 of the Sublease.

     9.   In the event Tenant is in default under any of the terms and
conditions of the Master Lease, Landlord may elect to receive directly from
Subtenant all sums due or payable to Tenant by Subtenant pursuant to the
Sublease, and upon receipt of Landlord's notice which states that Tenant is in
default under the Master Lease and which directs Subtenant to pay all such sums
directly to Landlord, Subtenant shall thereafter pay Landlord any sums becoming
due or payable under the Sublease, and Tenant shall receive from Landlord a
corresponding credit for such sums against any and all payments then due or
thereafter becoming due from Tenant. Neither the service of such written notice
nor the receipt of such direct payments shall cause Landlord to assume any of
Tenant's duties, obligations and/or liabilities under the
<PAGE>
 
Ms. Diana J. Sperle
Unisys Corporation
February 17, 1998
Page 4

Sublease, nor shall such event impose upon Landlord the duty or obligation to
honor the Sublease. Tenant hereby consents to the provisions of this Paragraph 9
and hereby indemnifies Subtenant, and holds Subtenant harmless, against and from
any and all liability, losses, costs, damages and expenses (including, but not
limited to, court costs and reasonable attorneys' fees) resulting from
Subtenant's fullfillment of its obligations under this Paragraph 9.

     10.  Tenant and Subtenant agree to indemnify and hold harmless Landlord and
its agents, including Landlord's management agent, against and from any loss,
cost, expense, damage or liability, including reasonable attorneys' fees,
incurred as a result of a claim by any person or entity (i) that it is entitled
to a commission, finder's fee or like payment in connection with the Sublease or
(ii) relating to or arising out of the Sublease or any related agreement or
dealing.

     11.  Landlord shall not be obligated to lease Subtenant any parking spaces
in the building's parking facilities, other than such parking spaces as Tenant
would be entitled to lease under the Master Lease, it being agreed that
Subtenant's rights pursuant to Paragraph 41 of the Proposed Sublease relate
solely to parking spaces currently leased by Tenant.

     12.  In accordance with Section 12.4 of the Master Lease, Tenant is
responsible for all actual attorney' fees incurred by Landlord in connection
with the Sublease; provided, however, that Tenant's responsibility for such
attorney' fees shall not exceed One Thousand Dollars ($1,000.00).

     13.  In addition to Subtenant's requirement, pursuant to Paragraph 9 of the
Proposed Sublease, to provide Tenant with all occupancy permits and certificates
as may be required for Subtenant's occupancy of the Subleased Premises prior to
Subtenant's occupancy thereof (such permits and certificates are hereinafter
collectively referred to as "Occupancy Permits"), Subtenant shall provide
photocopies of all Occupancy Permits to Landlord prior to Subtenant's occupancy
of the Subleased Premises.

     14.  In addition to Subtenant's requirement, pursuant to Paragraph 10 of
the Proposed Sublease, to provide Tenant with (i) partial releases of liens,
commencing with the second progress payment, and (ii) upon completion of any
Alterations, a final release of liens executed by all contractors and
subcontractors who performed such Alterations (the releases referred to in
clauses (i) and (ii) of this Paragraph are hereinafter collectively referred to
as "Lien Releases"), Subtenant, upon its receipt of such Lien Releases, shall
promptly provide photocopies of same to Landlord.
<PAGE>
 
Ms. Diana J. Sperle
Unisys Corporation
February 17, 1998
Page 5

     15.  Pursuant to Section 32 of the Master Lease, any placement of
Subtenant's signage on the Premises (as such term "Premises" is defined in the
Master Lease) shall be subject to Landlord's prior written consent, which may be
withheld in Landlord's reasonable discretion; if approved by Landlord, such
signage shall comply with applicable laws. In furtherance of the foregoing, and
not in limitation thereof:

          (i)       Landlord hereby approves the signage pursuant to Paragraph
                    40(a) of the Sublease and as shown on Exhibit B to this
                    letter agreement, provided that (a) Subtenant's right to
                    place its name on such sign monument at the main Sunrise
                    Valley Drive entrance shall be limited to the use of the
                    existing monument in its existing location (which Subtenant
                    shall have no right to relocate), (b) neither Tenant nor
                    Subtenant shall have any right to construct a second sign
                    monument at such main entrance to the Premises or to use any
                    new sign monument which may be constructed by Landlord, and
                    (c) the signage shown on Exhibit B hereto shall be no larger
                    than the signage existing in such location on the date
                    hereof, shall use the existing monument color, and the color
                    of lettering and logo on such sign shall, in the case of
                    Subtenant, be the same color of blue as is used in all of
                    Subtenant's exterior signage generally at premises which it
                    owns or leases (the "Subtenant's Standard Color") and, in
                    the case of Tenant, be the same color red as is used in all
                    of Tenant's exterior signage at the Premises, and

          (ii)      Landlord hereby approves the signage pursuant to Paragraph
                    40(b) and Exhibit E of the Sublease, in the location shown
                    on said Exhibit E, provided that (a) such signage shall (i)
                    be no larger than the signage existing in such location on
                    the date hereof, and (ii) use the existing monument color,
                    and (b) the color of Subtenant's lettering and logo on such
                    sign shall be the Subtenant's Standard Color. Subtenant
                    shall have no right to relocate the existing sign monument.
                    Notwithstanding the foregoing, Landlord hereby acknowledges
                    that Tenant intends to relocate the existing sign monument
                    closer to the building known as "Building #2", subject to
                    Landlord's reasonable approval as to its location. Landlord
                    hereby approves white lettering for Subtenant's name on the
                    glass doors to the Building.

     16.  Subtenant shall have no rights pursuant to Section 47 of the Master
          Lease.
<PAGE>
 
Ms. Diana J. Sperle
Unisys Corporation
February 17, 1998
Page 6 

     If the Tenant and Subtenant are in agreement with the foregoing, as
evidence of such agreement, please countersign and return a copy of this letter
to the undersigned,  Upon receipt of a fully executed countersigned copy of this
letter, Landlord shall be deemed to have granted its consent to the Sublease.

                                   Sincerely yours,
     
                                   TrizecHahn Reston Unisys I LLC, a 
                                   Virginia limited liability company


                                   By: /s/ Brian P. Coulter
                                      --------------------------------- 
                                   Name: Brian P. Coulter
                                        -------------------------------
                                   Its: Senior Vice President
                                       --------------------------------

                                   TrizecHahn Reston II LLC, a Virginia limited 
                                   liability company

                                   By: /s/ Brian P. Coulter
                                      ---------------------------------
                                   Name: Brain P. Coulter
                                        -------------------------------
                                   Its: Senior Vice President
                                       -------------------------------- 
<PAGE>
 
Ms. Diana J. Sperle
Unisys Corporation
February 17, 1998
Page 7

Accepted and Agreed:

Tenant:

UNISYS CORPORATION, a Delaware corporation

By: /s/ Gregory T. Fischer
   ----------------------------------------------------
     Its: Vice President- Facilities & Asset Management
         ----------------------------------------------

Subtenant:

PSINET, INC., a New York corporation

By: /s/ Harold S. Wills
   ---------------------------------------------------- 
     Its: Chief Operations Officer
         ---------------------------------------------- 
<PAGE>
 
                                                   (Attachment to Exhibit 10.10)

The following Exhibits and Schedules have been omitted, which the Company agrees
to furnish supplementally to the Commission upon request:

Exhibit A-Floor Plan
Exhibit C-Monthly Base Rent
Exhibit D- Janitorial Specifications
Exhibit E-Sinage
Exhibit F-Generators
<PAGE>
 
                                                                       Exhibit B

                                                                   Exhibit 10.10




                                 DEED OF LEASE
                                      AND
                               SPECIAL NET LEASE



                                    BETWEEN

                               UMT RESTON, INC.
                            A DELAWARE CORPORATION

                                      AND

                              UNISYS CORPORATION
<PAGE>
 
                                TABLE OF CONTENTS 
                                -----------------

<TABLE>
<CAPTION>
                                                                            Page
                                                                            ----
<S>          <C>                                                            <C>
SECTION 1    PARTIES....................................................     1

SECTION 2    PREMISES...................................................     1
             2.1   Land.................................................     1
             2.2   Improvements.........................................     1
             2.3   Fixtures.............................................     1
             2.4   Other Interests......................................     2

SECTION 3    TERM.......................................................     2
             3.1   Term.................................................     2

SECTION 4    RENT.......................................................     2
             4.1   Description..........................................     2
             4.2   Rent Adjustment......................................     3
             4.3   Rent.................................................     4

SECTION 5    RIGHT OF FIRST REFUSAL.....................................     4
             5.1   Right of First Refusal...............................     4
             5.2   Certificate..........................................     6

SECTION 6    USE........................................................     7
             6.1   Use..................................................     7
             6.2   Compliance with Law..................................     7
             6.3   Acceptance of Possession and
                   Condition of Premises................................     7

SECTION 7    MAINTENANCE, REPAIRS AND ALTERATIONS.......................     8
             7.1   Lessee's Obligations.................................     8
             7.2   Surrender............................................     9
             7.3   Lessor's Rights......................................    10
             7.4   Lessor's Obligations.................................    10
             7.5   Alterations, Improvements
                   and Additions........................................    11

SECTION 8    INSURANCE INDEMNITY........................................    13
             8.1   Insuring Party.......................................    13
             8.2   Liability Insurance..................................    14
             8.3   Property Insurance...................................    14
             8.4   Insurance Policies...................................    15
             8.5   Waiver of Subrogation................................    16
             8.6   Indemnity............................................    16
             8.7   Exemption of Lessor from Liability...................    16
</TABLE>

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TABLE OF CONTENTS (continued)
-----------------

<TABLE>
<CAPTION>
                                                                            Page
                                                                            ----
<S>          <C>                                                            <C>
SECTION 9    DAMAGE, DESTRUCTION, OBLIGATION TO
             REBUILD, RENT ABATEMENT....................................    17
             9.1 Obligation to Rebuild..................................    17
             9.2 Insurance Proceeds.....................................    18
             9.3 Damage Near End of Term................................    18
             9.4 No Abatement of Rent...................................    19
             9.5 Termination-Advance Payments...........................    19
             9.6 Waiver.................................................    20

SECTION 10   REAL PROPERTY TAXES........................................    20
             10.1 Payment of Taxes......................................    20
             10.2 Definition of "Real Property Tax......................    21
             10.3 Personal Property Taxes...............................    23

SECTION 11   UTILITIES..................................................    23

SECTION 12   ASSIGNING AND SUBLETTING...................................    23
             12.1 Lessor's Consent Required.............................    23
             12.2 Lessee Affiliate......................................    24
             12.3 No Release of Lessee..................................    24
             12.4 Attorneys' Fees.......................................    25

SECTION 13   DEFAULTS; REMEDIES.........................................    25
             13.1 Defaults..............................................    25
             13.2 Remedies..............................................    27
             13.3 Default by Lessor.....................................    29
             13.4 Late Charges..........................................    29
             13.5 Impounds..............................................    30

SECTION 14   CONDEMNATION...............................................    30

SECTION 15   ESTOPPEL STATEMENT.........................................    32
             15.1 Lessee Estoppel.......................................    32
             15.2 Failure to Deliver Lessee Estoppel....................    32
             15.3 Financial Information.................................    32
             15.4 Lessor Estoppel.......................................    32

SECTION 16   LESSOR'S LIABILITY.........................................    33

SECTION 17   SEVERABILITY...............................................    33

SECTION 18   INTEREST ON PAST-DUE OBLIGATIONS...........................    33
</TABLE>

                                    --ii--
<PAGE>
 
TABLE OF CONTENTS (continued)
----------------- 

<TABLE> 
<CAPTION> 
                                                                 Page
                                                                 ----
<S>           <C>                                                <C> 
SECTION 19    TIME OF ESSENCE..................................... 34

SECTION 20    ADDITIONAL RENT..................................... 34

SECTION 21    INCORPORATION OF PRIOR AGREEMENTS;
              AMENDMENTS.......................................... 34

SECTION 22    NOTICES............................................. 35

SECTION 23    WAIVERS............................................. 35

SECTION 24    HOLDING OVER........................................ 35

SECTION 25    CUMULATIVE REMEDIES................................. 36

SECTION 26    COVENANTS AND CONDITIONS............................ 36

SECTION 27    BINDING EFFECT; CHOICE OF LAW....................... 36

SECTION 28    SUBORDINATION....................................... 36
              28.1 Generally...................................... 36
              28.2 Execution of Documents......................... 37

SECTION 29    ATTORNEY'S FEES..................................... 37

SECTION 30    LESSOR'S ACCESS..................................... 37

SECTION 31    AUCTIONS............................................ 38

SECTION 32    SIGNS............................................... 38

SECTION 33    MERGER.............................................. 38

SECTION 34    GUARANTOR........................................... 39

SECTION 35    QUIET POSSESSION.................................... 39

SECTION 36    MULTIPLE-TENANT BUILDING............................ 39

SECTION 37    SECURITY MEASURES................................... 40

SECTION 38    EASEMENTS........................................... 40

SECTION 39    PERFORMANCE UNDER PROTEST........................... 40
</TABLE>

                                    - iii -
<PAGE>
 
TABLE OF CONTENTS (continued)
-----------------            

<TABLE>
<CAPTION>
                                                                 Page
                                                                 ----
<S>           <C>                                                <C>
SECTION 40    AUTHORITY........................................... 41

SECTION 41    CONFLICT............................................ 41

SECTION 42    OPTIONS............................................. 41
              42.1 Definition..................................... 41
              42.1 Assignment..................................... 41
              42.3 Multiple Options............................... 41
              42.4 Effect of Default on Options................... 41

SECTION 43    OPTIONS OF LESSOR AND LESSEE........................ 42
              43.1 Addition Purchase Option....................... 42
              43.2 Lessee's Lease Extension Options............... 44
              43.3 Qualifications of Options...................... 45

SECTION 44    DETERMINATION AND FAIR
              MARKET RENTAL VALUE................................. 45
              44.1 Fair Market Rental Value....................... 45
              44.2 Arbitration Procedures......................... 45

SECTION 45    HAZARDOUS MATERIALS................................. 47
              45.1 Covenants of Lessee............................ 47
              45.2 Hazardous Material Definition.................. 50
              45.3 Governmental Regulations Defined............... 51
              45.4 Underground Storage Tanks...................... 51
              45.5 Groundwater Monitoring......................... 51
              45.6 Survival....................................... 52

SECTION 46    LIENS............................................... 52

SECTION 47    LESSOR'S RIGHT TO DEVELOP EXCESS LAND............... 52
              47.1 Release of Excess Land......................... 52
              47.2 Development of Excess Land..................... 52

SECTION 48    FOREIGN BUYERS...................................... 53
              48.1 Lessee's right to Consent...................... 53
              48.2 Lessor's Obligations........................... 54

SIGNATURES    .................................................... 55
</TABLE> 
                                     -iv-
<PAGE>
 
TABLE OF CONTENTS (continued)
-----------------            

<TABLE>
<CAPTION>
                                                           Page of     
EXHIBITS                                               First Reference 
----------                                             ---------------  
<S>            <C>                                     <C> 
 
"A"            Legal Description                               1    
"B"            Special Improvements                            2
"C"            Released Land                                  52 
</TABLE>

                                      -v-
<PAGE>
 
                                 DEED OF LEASE
                                      AND
                               SPECIAL NET LEASE

                                   SECTION 1

                                    PARTIES

          THIS DEED OF LEASE, dated, for reference purposes only, as of March
29, 1990, is made by and between UMT RESTON, INC., a Delaware corporation
(herein called "Lessor") and UNISYS CORPORATION, a Delaware corporation (herein
called "Lessee").

          This Lease is entered into pursuant to that certain Agreement of
Purchase and Sale dated as of March 29, 1990 ("Purchase Agreement") between
Lessee, as Seller, and Lessor, as Buyer, for the purpose of leasing back to
Lessee the property acquired by Lessor pursuant to the Purchase Agreement.


                                   SECTION 2

                                   PREMISES

          Lessor hereby demises and leases to Lessee and Lessee leases from
Lessor for the term, at the rental, and upon all of the conditions set forth
herein, that certain property (collectively, "Premises") described herein:

          2.1  Land. The land located at 12010 Sunrise Valley Drive, Reston,
               ----
County of Fairfax ("County"), Commonwealth of Virginia ("State"), more
particularly described on Exhibit "A" attached hereto (the "Land").
                          ----------

          2.2  Improvements. All buildings, parking facilities and other
               ------------   
improvements now on the Land (the "Improvements").

          2.3  Fixtures. All fixtures and trade fixtures attached to or
               --------
appurtenant to the Improvements, including but not limited to the following: the
heating, ventilation and air conditioning systems, which include all boilers,
pumps, compressors, chillers, cooling towers, ducting and hot and cold water
plumbing, whether located in the interior or exterior walls, floors and ceilings
of the Improvements, on the rooftop of the Improvements or on the Land outside
the Improvements; the plumbing systems, which includes all piping, hot and cold
water systems, sewer and water supply and plumbing fixtures; any cafeteria
fixtures; the electrical system, which includes all transformers, circuitry and
wiring; all lighting fixtures; and any fire alarm and sprinkler systems.
Notwithstanding anything to the contrary contained herein, the parties agree and
<PAGE>
 
acknowledge that those certain fixtures and improvements ("Special
Improvements") described on Exhibit "B". attached hereto were not purchased by
                            ----------
Lessor pursuant to the Purchase Agreement and remain the property of Lessee.
Based thereupon, the Special Improvements do not constitute a portion of the
Premises for purposes of this Lease.

          2.4  Other Interests. All right, title and interest of the Lessor in
               ---------------
and to any easements, rights-of-way or other interest in, on, or to any land,
highway, street, road or avenue open or proposed in, on, across, in front of,
abutting or adjoining the Land.


                                   SECTION 3

                                     TERM

          3.1  Term. The term of this Lease shall be for fifteen (15) years,
               ----
with four (4) five (5) year options to extend in accordance with Paragraph 43.2
(which extensions, when made, shall be deemed part of the term of this Lease).
The term shall commence on March 30, 1990 (the "Commencement Date") and shall
end on the last day of the calendar month in which the fifteenth (15th)
anniversary falls (subject to extension in accordance with Paragraph 43.2).


                                   SECTION 4

                                     RENT
                                     ----   
          4.1  Description. Lessee shall pay to Lessor or as Lessor shall direct
               -----------
as rent for the Premises, monthly rental ("Monthly Basic Rent") in the amount of
$ _____________, as adjusted pursuant to Paragraph 4.2, in advance, on the first
day of each month of the term hereof. Rent for any period during the term hereof
which is less than one (l) month shall be a pro rata portion of the Monthly
Basic Rent (based on the number of days in such month), as adjusted. Rent shall
be payable without deduction or offset of any kind to Lessor at the address
stated herein or to such other persons or at such other places as Lessor may
from time to time designate in writing. Lessor or Lessor's agent shall, on a
monthly basis, ten (10) days before the next installment of Monthly 8asic Rent
is due, send Lessee a written statement stating the amount of the installment of
Monthly Basic Rent next due, but Lessee shall still be obligated to pay the rent
therein specified to be paid notwithstanding Lessor's failure to send such
notice at the time specified.

                                      -2-
<PAGE>
 
          4.2  Rent Adjustment. The Monthly Basic Rent shall be adjusted as
               ---------------
follows:

          (a)  Commencing on the first day of the second Lease Year (as
     hereinafter defined) and, subject to the terms of Section 4.2(b), on the
     first day of every Lease Year thereafter during the term of this Lease, the
     Monthly Basic Rent shall be increased by ________________ percent (%). For
     purposes of this Lease, the term "Lease Year" shall mean each twelve-month
     period during the term of this Lease commencing on April 1 and ending on
     the following March 31. Notwithstanding the immediately preceding sentence,
     in no event shall the Monthly Basic Rent be increased on the first day of
     any Lease Year in which the Monthly Basic Rent is increased pursuant to
     Paragraph 4.2(b) unless the Monthly Basic Rent which would be in effect
     were such Monthly Basic Rent increased pursuant to this Paragraph 4.2(a)
     exceeds the Monthly Basic Rent which would be in effect were such Monthly
     Basic Rent increased pursuant to Paragraph 4.2(b).

          (b)  On the first day of April, 1995 and on the first day of April,
     2000 (each, an "Adjustment Date"), the Monthly Basic Rent shall be
     increased as set forth in this Paragraph 4.2(b), but only to the extent
     that the Monthly Basic Rent, as so increased pursuant to this Paragraph
     4.2(b), exceeds the Monthly Basic Rent which would otherwise be in effect
     were the Monthly Basic Rent increased on such adjustment date as set forth
     in Paragraph 4.2(a). Such rent increase shall be based on the increase, if
     any, in the Consumer Price Index for All Items, All Urban Consumers for the
     U.S. City Average (1982-84-100) ("CPI") published monthly by the United
     States Department of Labor Bureau of Labor Statistics ("Index"), for the-
     Comparison Month (described below) in comparison to the Index published for
     December, 1989 ("Base Month"). The Base Month Index shall be compared with
     the Index for the calendar month four (4) months prior to the Adjustment
     Date ("Comparison Month"). If the Index for any Comparison Month is 7
     higher than the Base Month Index, then, subject to the terms of this
     Paragraph 4.2, the Monthly Basic Rent next due shall equal the Monthly
     Basic Rent due for April, 1990 increased by a percentage which shall be
     calculated by dividing the Base Month Index into that number which
     represents the difference between the Base Month Index and the Index for
     such Comparison 

                                      -3-
<PAGE>
 
     Month ("Percentage Rent Increase"); provided, however, that in no event
                                         --------  -------
     shall such percentage exceed _______% for the adjustment to be made at the
     end of the fifth Lease Year and ________% for the adjustment to be made at
     the end of the tenth Lease Year. If the Index for any Comparison Month is
     lower than the Base Month Index, then the terms of this Paragraph 4.2(b)
     shall not apply to such Adjustment Date.

Should said Bureau discontinue the publication of the above Index, or publish
the same less frequently, or alter the same in some other manner, then Lessor
and Lessee shall reasonably agree upon adoption of a substitute index or
substitute procedure which reasonably reflects and monitors consumer prices. If
Lessor and Lessee are unable to agree on such substitute index or procedure by
the "Specified Date" (as defined in Paragraph 44.2), such matter shall be
decided by arbitration in accordance with Section 44 hereof.

          4.3  Rent. Monthly Basic Rent and the amounts required to be expended
               ----
by Lessee pursuant to Paragraphs 7.1 and 9.1 hereof and elsewhere in this Lease
sometimes are hereinafter collectively referred to as "Rent."


                                   SECTION 5

                            RIGHT OF FIRST REFUSAL

                                      -4-
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                          [Page intentionally blank]





                                      -5-
<PAGE>
 


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                                      -6-
<PAGE>
 
                                   SECTION 6

                                      USE

          6.1  Use.  The Premises shall be used consistently with any legal and
               ---                                                             
permitted uses in accordance with applicable zoning ordinances.

          6.2  Compliance with Law.
               ------------------- 

               (a)  Lessor makes no warranty to Lessee regarding whether the
          Premises violate any covenants or restrictions of record, or any
          applicable building or zoning code, regulation or ordinance in effect
          during the term of this Lease.

               (b)  Lessee shall, at Lessee's expense, comply promptly with all
          applicable statutes, ordinances, rules, regulations, orders, covenants
          and restrictions of record, and requirements in effect during the term
          or any part of the term hereof, whensoever arising, regulating the use
          by Lessee of the Premises (or any portion thereof). Lessee shall not
          use or permit the use of the Premises (or any portion thereof) in any
          manner that will tend to create waste (as determined under applicable
          law) or a nuisance or which shall tend to disturb any other tenant.

          6.3  Acceptance of Possession and Condition of Premises. Lessee's
               --------------------------------------------------          
acceptance of possession of the Premises on the commencement of the term shall
constitute acceptance by Lessee of the Premises in its then existing, "AS-IS"
condition, subject to all applicable zoning, municipal, county and state laws,
ordinances and regulations governing and regulating the use or condition of the
Premises and all applicable covenants or restrictions or record. Lessee
acknowledges that neither Lessor, nor any person acting on behalf of Lessor, has
made any representation or warranty whatsoever concerning the condition of the
Premises or the present or future suitability of the Premises for the conduct of
Lessee's business.

               Lessee acknowledges that (a) it was the owner of the Premises
prior to the Commencement Date and it accepts the Premises in the condition and
with all defects and contamination, if any, existing as of the Commencement
Date, and (b) Lessee shall have complete responsibility for compliance with all
laws, orders, regulations and requirements with respect to the use of the
Premises by Lessee. Lessor and Lessee 

                                      -7-
<PAGE>
 
understand and agree that the provisions of this Paragraph 6.3 shall survive the
expiration or sooner termination of this Lease.


                                   SECTION 7

                     MAINTENANCE, REPAIRS AND ALTERATIONS

          7.1  Lessee's Obligations. As used in this Lease, the term "repair"
shall include all necessary or appropriate replacements and all expenditures for
capital repair, maintenance and replacement, including, but not limited to, any
work with respect to the roof as set forth in Paragraph 45.1(c)(i) hereof.
Lessee's obligation to maintain the Premises shall include the obligation to
make all necessary or appropriate capital expenditures to repair and replace
structural elements of the Premises such as, but not limited to, the floors,
walls, ceilings and roofs. Lessee shall keep in good order, condition and repair
(including all necessary or advisable capital replacements, including roof and
HVAC), the Premises and every part thereof, structural and non-structural
(whether or not such portion of the Premises requiring repair or the means of
repairing the same are reasonable or readily accessible to Lessee, and whether
or not the need for such repairs or replacement occurs as a result of Lessee's
use, prior use, the elements or the age of such portion of the Premises),
including, without limiting the generality of the foregoing, all plumbing,
heating, air conditioning, ventilating, electrical, lighting facilities and
equipment within the Premises, fixtures, walls (interior and exterior),
foundations, ceiling, roofs (interior and exterior), floors, windows, doors,
plate glass and skylights located within the Premises, and all landscaping,
driveways, parking lots, fences and signs located on the Premises, and to the
extent that it or Lessor has or may have the legal obligation to do so,
sidewalks and parkways adjacent to the Premises. In particular, and not in
limitation of the terms of the immediately preceding sentence, Lessee will
properly maintain in good repair the Phase 1 building boilers referenced in the
Kellermeyer Dodfryt Hart, P.C. Engineering Report dated February 23, 1990, the
-----------------------------                           -----------------
Phase 1 building roof and the parking lot surfaces on the Property, all at
Lessee ts expense. Within six (6) months after the date of this Lease, Lessee
shall remove all flue pipe gaskets in the Phase 1 building mechanical penthouse
which contain asbestos and properly dispose of same in accordance with
applicable law, all at Lessee's expense. In the event Lessee performs repair or
replacement work with respect to the roof, Lessee also agrees to perform all
asbestos removal which may be necessitated by any such work, at its sole cost
and expense. Except in the case of 

                                      -8-
<PAGE>
 
an emergency (and then only to the extent of such emergency), Lessee shall not
perform any particular repair work without Lessor's prior written approval where
such repair shall involve material alterations to the structural portions of the
Improvements; the roof, the external facades of the Improvements or any building
systems (e.g. air conditioning, heating, ventilation, electrical and mechanical
         ---
systems) provided however, if Lessor unconditionally refuses to approve such
repair, Lessee's repair obligation shall be excused and the failure to make such
repair shall not be a default hereunder. At the end of each year during the term
of this Lease, as may be extended, Lessee shall promptly deliver to Lessor a
report on the condition of the Premises, which report shall include a summary of
all repairs performed during such year in excess of $100,000.00. Lessee's
maintenance obligations shall specifically extend to the clean-up of all
"Hazardous Materials. (as defined in Paragraph 45.2 below) occurring in, on or
about the Premises.

          7.2  Surrender. No earlier than twelve (12) months nor later than six
               ---------
(6) months prior to the expiration or sooner termination of the term hereof or
any extension term, as applicable, Lessee shall give Lessor notice
("Expiration/Termination Notice") of all alterations, improvements and additions
contained within the Premises as of the date thereof. Within thirty (30) days
after the date of such Expiration/Termination Notice, Lessor shall give Lessee
Notice of the alterations improvements and/or additions which Lessee shall be
required to remove as of the expiration or sooner termination of this Lease
provided that either (a) Lessor indicated in its consent to such alteration,
improvement or addition that Lessor reserved its right to require such removal
or (b) Lessee failed to obtain Lessor's consent to such alteration, improvement
or addition. On the last day of the term hereof, or any sooner termination,
Lessee shall surrender the Premises to Lessor in the same condition as when
received ordinary wear and tear excepted, free and clear of debris; provided
that Lessee shall have removed all "Hazardous Materials" (as defined in
Paragraph 45.2) from the Premises and all Special Improvements from the Premises
(which Lessor requires Lessee to remove) in a manner which complies with all
"Governmental Regulations" (as defined in Paragraph 45.3); provided, further
that Lessee shall be required to remove all other fixtures, tenant improvements,
equipment and furnishings which Lessor requires, within sissy (60) days after
the delivery of Lessor's notice to so remove. Lessee shall repair any damages to
the Premises occasioned by the installation or removal of the Special
Improvements and Lessee's tenant improvements, trade fixtures, furnishings and
equipment or occasioned by Lessee's removal of all Hazardous Materials. 

                                      -9-
<PAGE>
 
Except for the removal of all Hazardous Materials, if the cost of repairing the
damage to the Premises occasioned by any other removal would be prohibitive, in
Lessee's reasonable determination, Lessee shall deliver written notice thereof
to Lessor within 10 days after the Lessor's notice to remove. In such event,
Lessor shall then advise Lessee in writing within 10 days after receipt of
Lessee's notice to either (a) remove the subject improvements up to but not
including the shell portion of the Premises, or (b) leave the Premises as they
are, or (c) remove such improvements without repair to the Premises.
Notwithstanding anything to the contrary otherwise stated in this Lease, Lessee
shall leave the air lines, power panels, electrical distribution systems,
lighting fixtures, space heaters, air conditioning, plumbing and fencing on the
Premises in good operating condition, normal wear and tear excepted.

          7.3  Lessor's Rights.  If Lessee fails to perform Lessee's obligations
               ---------------                                                  
under Paragraph 9 (relating to destruction of the Premises), or under any other
paragraph in this Lease within fifteen (15) days after Lessor's written notice
to Lessee of such failure or, if such obligations cannot be performed within
such time period (taking into account the possible need to prepare plans, obtain
bids, obtain permits or permit fabrication by contractors), Lessee has not
commenced to perform in due diligence and expedience and does not diligently
prosecute such obligations to completion, Lessor may at its option (but shall
not be required to) enter upon the Premises after reasonable notice to Lessee
and compliance with then-applicable governmental security requirements and
Lessee's reasonable rules and regulations with respect to classified areas
(except in the case of an emergency, in which case no notice shall be required),
perform such obligations on Lessee's behalf and put the same in good order,
condition and repair, and the cost thereof together with interest thereon at the
maximum rate then allowable by law shall become due and payable as additional
rent to Lessor together with Lessee's next rent installment.

          7.4  Lessor's Obligations.  Except for the obligations of Lessor under
               --------------------                                             
Paragraph 9 (relating to destruction of the Premises) and under Paragraph 14
(relating to condemnation of the Premises), it is intended by the parties hereto
that Lessor have no obligation, in any manner whatsoever, to repair or maintain
the Premises nor the buildings located thereon nor the equipment therein,
whether structural or non-structural, all of which obligations are intended to
be that of the Lessee under Paragraph 7.1 hereof. Lessee expressly waives, to
the extent permitted under applicable law, the benefit of any statute now or
hereinafter in effect which would 

                                     -10-
<PAGE>
 
otherwise afford Lessee the right to make repairs at Lessor's expense or to
terminate this Lease because of Lessor's failure to keep the Premises in good
order, condition and repair.

          7.5  Alterations Improvements and Additions.
               -------------------------------------- 

               (a)  Lessee shall not have the right to make any alteration,
          improvement or addition to that part of the Premises which is Excess
          Land (as defined in Paragraph 47.1). Subject to the foregoing, Lessee
          shall not, without Lessor's prior written consent, which consent shall
          not unreasonably be withheld, make any alterations, improvements or
          additions to the Premises or the building structures, including but
          not limited to the walls (exterior and interior) and roof (exterior
          and interior); to the exterior appearance of the buildings; or to the
          utilities and building systems servicing the Premises and shall not
          take any action or make any filings with Fairfax County or any other
          governmental entity which would affect the zoning of, or development
          rights related to, the Premises. Notwithstanding the foregoing,
          Lessee may make any non-structural alterations, improvements and
          additions to the Premises without Lessor's prior written consent
          including "Utility Installations" (which shall mean carpeting, window
          coverings, air lines, power panels, electrical distribution systems,
          lighting fixtures, space heaters, air conditioning, plumbing and
          fencing); provided, that (i) the cost of constructing said
          alterations, improvements and additions do not exceed $100,000.00 per
          alteration (which amount may be adjusted by the parties hereto in
          writing from time to time), (ii) said alterations do not materially
          alter the ratio of office and non-office space contained in the
          Premises or the general ratio of uses of the Premises, and (iii)
          Lessee promptly provides to Lessor after completion thereof "as-built"
          plans and specifications of the alterations. Lessor may require that
          Lessee remove any or all of said alterations, improvements or
          additions costing in excess of $100,000.00 (which amount may be
          adjusted in writing by mutual agreement of the parties hereto from
          time to time), Lessor may require Lessee to provide Lessor, at
          Lessee's sole cost and expense, a lien and completion bond in an
          amount equal to one and one-half times the estimated cost of such
          improvements, to insure Lessor against any liability for mechanics'
          and materialmen's liens and to insure completion of the work. Except
          for those items not requiring Lessor's approval, should

                                     -11-
<PAGE>
 
          Lessee make any alterations, improvements or additions of the types
          specified hereunder, without the prior written approval of Lessor,
          Lessor may require that Lessee remove any or all of the same.

               (b)  Any alterations, improvements or additions to the Premises
          that Lessee shall desire to make and which require the consent of the
          Lessor shall be presented to Lessor in written form, with proposed
          detailed plans. If Lessor shall give its consent, the consent shall be
          deemed conditioned upon Lessee acquiring a permit to do so from
          appropriate governmental agencies, the furnishing of a copy thereof to
          Lessor prior to the commencement of the work and the compliance by
          Lessee with all conditions of said permit in a prompt and expeditious
          manner. Subject to the provisions of this Paragraph 7.5, Lessor agrees
          to cooperate with Lessee in connection with any alteration,
          improvement or addition to the Premises by Lessee including, without
          limitation granting necessary ingress/egress, utility and/or signage
          easements that do not unreasonably interfere with the use of the
          Excess Land by Lessor, provided that Lessor shall not be required to
          expend any funds in connection therewith. Lessee shall have the right
          to two access points to Sunrise Valley Drive of similar quality to the
          access as it exists on the Commencement Date. Lessee shall have the
          right to signage similar to that existing on the Commencement Date,
          subject to governmental regulations, provided that Lessee shall not be
          required to reduce such signage in order to accommodate signage on
          Released Land (as defined in Paragraph 47.1).

               (c)  Lessee shall pay, when due, all claims for labor or
          materials furnished or alleged to have been furnished to or for Lessee
          at or for use in the Premises, which claims are or may be secured by
          any mechanics' or materialmen's lien against the Premises or any
          interest therein. In addition to obtaining Lessor's consent pursuant
          to this Paragraph 7.5, Lessee shall give Lessor not less than ten (10)
          days notice prior to the commencement of any work in the Premises
          reasonably estimated to cost in excess of $250,000.00, and Lessor
          shall have the right to post notices of non-responsibility in or on
          the Premises as provided by law. If Lessee shall in good faith
          contest the validity of any such lien, claim or demand, then Lessee
          shall, at its sole expense, defend itself and Lessor against the same
          and shall pay and satisfy any such adverse judgment that may be
          rendered

                                     -12-
<PAGE>
 
     thereon before the enforcement thereof against the Lessor or the Premises,
     upon the condition that if Lessor shall require, Lessee shall furnish to
     Lessor a surety bond satisfactory to Lessor in an amount equal to such
     contested lien, claim or demand indemnifying Lessor against liability for
     the same and holding the Premises free from the effect of such lien, claim
     or demand. Lessor may require Lessee to pay Lessor's attorneys fees and
     costs in participating in such action if Lessor shall decide it is to its
     best interest to do so.

          (d)  Unless Lessor requires their removal, as set forth in Paragraph
     7.5(a), all alterations, improvements (including standard office partitions
     and suspended tile ceilings), additions and Utility Installations (whether
     or not such Utility Installations constitute trade fixtures of Lessee)
     which may be made on the Premises, shall become the property of Lessor and
     remain upon and be surrendered with the Premises at the expiration of the
     term. Notwithstanding the provisions of this Paragraph 7.5(d), Lessee's
     machinery and equipment, other than that which is affixed to the Premises
     so that it cannot be removed without material damage to the Premises
     (except if Lessee demonstrates that the same can be removed by Lessee and
     such damage repaired and restored to Lessor's reasonable satisfaction at
     Lessee's sole cost and expense, and Lessee makes such repair and
     restoration) and other than that which is described as a fixture in
     Paragraph 2.3 and thus is a component of the Premises, shall remain the
     property of Lessee and may be removed by Lessee subject to the provisions
     of Paragraph 7.2.

          (e)  Upon Lessee's completion of any work of alteration, improvement
     or addition described herein, Lessee shall promptly deliver to Lessor
     copies of all plans and specifications prepared in connection with such
     work of alteration, improvement or addition.

                                   SECTION 8

                             INSURANCE; INDEMNITY

     8.1  Insuring Party.  Lessee shall, as additional rent for the Premises,
          --------------                                                     
pay the cost of all insurance required hereunder.

                                     -13-
<PAGE>
 
     8.2  Liability Insurance.  Lessee shall at Lessee's expense obtain and keep
          -------------------                                                   
in force during the term of this Lease a policy of combined single limit, bodily
injury and property damage insurance insuring Lessor and Lessee against any
liability arising out of the use, occupancy or maintenance of the Premises and
all areas appurtenant thereto. Such insurance shall be a combined single limit
policy in an amount not less than $5,000,000 per occurrence which names Lessor
as an additional insured thereunder. The policy shall insure performance by
Lessee of the indemnity provisions of this Paragraph 8. The limits of said
insurance shall not, however, limit the liability of Lessee hereunder. Lessee
shall deliver to Lessor evidence satisfactory to Lessor of the timely payment by
Lessee of the premiums for such insurance.

     8.3  Property Insurance.
          ------------------ 

          (a)  Lessee shall obtain and keep in force during the term of this
     Lease policy or policies of insurance covering loss or damage to the
     Premises, in the amount of the full replacement value thereof including all
     improvements, as the same may exist from time to time, but in no event less
     than the total amount required by Lessor from time to time, against all
     perils included within the classification of fire, extended coverage,
     vandalism, malicious mischief, flood, earthquake, and special extended
     perils ("tall risk" as such term is used in the insurance industry). A
     stipulated value or agreed amount endorsement deleting the coinsurance
     provision shall be procured with said insurance. Said insurance policy or
     policies must be approved by the Lessor, whose approval will not be
     unreasonably withheld. Said insurance shall name Lessor as a named insured
     and provide for payment of loss thereunder to Lessor or to the holders of
     mortgages or deeds of trust on the Premises as their interests may appear.
     The Lessee shall, in addition, obtain and keep in force during the term of
     this Lease a policy of business interruption insurance covering a period of
     one year, with loss payable to Lessee, which insurance shall also cover all
     rental payments, real estate taxes and insurance costs for said period. An
     automatic increase in insurance endorsement causing the increase in annual
     property insurance coverage by two percent (2%) per quarter will be
     required only if Lessee no longer maintains a replacement cost insurance
     policy. If the Lessee shall fail to procure and maintain said insurance the
     Lessor may, but shall not be required to, procure and maintain the same,
     but at the expense

                                     -14-
<PAGE>
 
     of Lessee. If such insurance coverage has a deductible clause, the
     deductible amount shall not exceed $250,000.00 per occurrence or some other
     limit as is mutually agreed upon between Lessor and Lessee from time to
     time, and Lessee shall be liable for such deductible amount.

          (b)  If the Premises are a part of a larger building, or if the
     Premises are part of a group of buildings owned by Lessor which are
     adjacent to the Premises, then Lessee shall pay for any increase in the
     property insurance of such other building or buildings if said increase is
     caused by Lessee's acts, omissions, use or occupancy of the Premises.

     8.4  Insurance Policies.  Insurance required hereunder shall be with
          ------------------                                             
companies (which may be part of an association of companies) admitted in the
State in which the Premises are located and holding a "General Policyholders
Rating'' of at least A and a financial rating of twelve (12) or better, or such
other rating as may be required by a lender having a lien on the premises, as
set forth in the most current issue of "Best's Insurance Guide". In lieu of
separate policies, the Lessee may maintain blanket or umbrella policies if such
policies provide the same coverage required herein with protection against each
risk not reducible by claims for other risks to amounts less than that specified
herein. The Lessee shall deliver to Lessor certificates of such insurance
required hereunder (evidencing that the insurance requirements hereof have been
met) promptly upon Lessee's receipt of each renewal thereof. Promptly upon the
execution of this Lease, Lessee shall deliver to Lessor certificates of
insurance evidencing the existence and amounts of such insurance and the
compliance of such insurance with the requirements of Section 8. No such policy
shall be cancellable or subject to reduction of coverage or other modification
except after thirty (30) days prior written notice by the insurer to Lessor.
Thirty (30) days prior to the expiration of such policies and upon Lessor's
request, Lessee shall furnish Lessor with renewals or "binders" thereof, or
Lessor may order such insurance and charge the cost thereof to Lessee, which
amount shall be payable by Lessee upon demand. Lessee shall not do or permit to
be done anything which shall invalidate the insurance policies referred to in
Paragraph 8. If Lessee does or permits to be done anything which shall increase
the cost of the insurance policies referred to in Paragraph 8.3, then Lessee
shall pay such additional premium or forthwith upon Lessor's demand reimburse
Lessor for any additional premiums attributable to any act or omission or
operation of Lessee causing such increase in the cost of insurance.

                                     -15-
<PAGE>
 
     8.5  Waiver of Subrogation.  Lessee and Lessor each hereby release and
          ---------------------                                            
relieve the other, and waive their entire right of recovery against the other
for loss or damage arising out of or incident to the perils insured against
under Paragraph 8.3, which perils occur in, on or about the Premises, whether
due to the negligence of Lessor or Lessee or their agents, employees,
contractors and/or invitees. Lessee and Lessor shall, upon obtaining the
policies of insurance required hereunder, give notice to the insurance carrier
or carriers that the foregoing mutual waiver of subrogation is contained in this
Lease and shall procure the consent of such carrier or carriers to the foregoing
mutual waiver of subrogation. All policies of insurance required hereunder shall
include a clause or endorsement denying the insurer any rights of subrogation
against the other party to the extent such rights are waived hereunder or have
otherwise been waived by the insured prior to the occurrence of injury or loss.

     8.6  Indemnity.  Lessee shall indemnify and hold harmless Lessor from and
          ---------                                                           
against any and all claims arising from Lessee's use of the Premises or from the
conduct of Lessee's business or from any activity, work or things done,
permitted or suffered by Lessee in or about the Premises or elsewhere and shall
further indemnify and hold harmless Lessor from and against any and all claims
arising from any breach or default in the performance of any obligation on
Lessee's part to be performed under the terms of this Lease, including Lessee 
covenants with respect to use, management, transportation or other activities
involving Hazardous Materials, or arising from any negligence of the Lessee, or
any of Lessee's agents, contractors, or employees, and from and against all
costs, attorneys' fees, expenses and liabilities incurred in the defense of any
such claim or any action or proceeding brought thereon, and in case any action
or proceeding be brought against Lessor by reason of any such claim, Lessee upon
notice from Lessor shall defend the same at Lessee's expense by counsel
satisfactory to Lessor. Lessee, as a material part of the consideration to
Lessor, hereby assumes all risk of damage to property or injury to persons, in,
upon or about the Premises arising from any cause and Lessee hereby waives all
claims in respect thereof against Lessor. Lessee shall be entitled to timely
notice and reasonable cooperation from the Lessor, as well as to control of the
defense and settlement of all such claims.

     8.7  Exemption of Lessor from Liability.  Lessee hereby agrees that Lessor
          ----------------------------------                                   
shall not be liable for injury to Lessee's business or any loss of income
therefrom or for damage to the goods, wares, merchandise or other property of
Lessee,

                                     -16-
<PAGE>
 
Lessee's employees, customers, or any other person in or about the Premises, nor
shall Lessor be liable for injury to the person of Lessee, Lessee's employees,
agents or contractors, whether such damage or injury is caused by or results
from fire, steam, electricity, gas, water or rain, or from the breakage,
leakage, obstruction or other defects of pipes, sprinklers, wires, appliances,
plumbing, air conditioning or lighting fixtures, or from any other cause,
whether the said damage or injury results from conditions arising upon the
Premises or upon other portions of the buildings of which the Premises are a
part, or from other sources or places and regardless of whether the cause of
such damage or injury or the means of repairing the same is accessible to
Lessee. Lessor shall not be liable for any damages arising from any act or
neglect of any other tenant, if any, of the buildings in which the Premises are
located.

                                   SECTION 9

          DAMAGE, DESTRUCTION, OBLIGATION TO REBUILD, RENT ABATEMENT

     9.1  Obligation to Rebuild.    In the event that some or all of the
          ---------------------                                         
improvements constituting a part of the Premises or the Premises itself are
damaged or destroyed, partially or totally, from an insured loss or earthquake
or a loss which would have been insured but for the actions or failure to act of
Lessee, its agents, employees or invitees, or from an uninsured loss which would
cost less than One Million Dollars ($1,000,000. 00) to repair, as reasonably
determined by an independent licensed architect retained by Lessor, then Lessee
shall repair, restore and rebuild the Premises to its condition existing
immediately prior to such damage or destruction and this Lease shall remain in
full force and effect. Such repair, restoration and rebuilding (all of which are
herein individually and collectively called "repair") shall be commenced within
a reasonable time after such damage or destruction has occurred and shall be
diligently pursued to completion. Lessee shall pay all costs of such repair in
excess of the available insurance proceeds. The appearance of "Hazardous
Material" (as defined in Paragraph 45.2 below) shall not be deemed an occurrence
of damage or destruction which is subject to the terms of this Section.

          In the event of an uninsured loss for which Lessee does not have the
obligation to repair, as set forth above, Lessor shall have the option, at its
sole discretion, to contribute the uninsured amount and repair or contribute the
uninsured amount and require Lessee to repair the Premises, in which event
Lessee's obligation to pay rent shall not terminate

                                     -17-
<PAGE>
 
during the period of reconstruction. Lessor shall notify Lessee of its election
to require such repair within thirty (30) days after the date of occurrence of
such damage. In the event Lessor elects not to repair the Premises, this Lease
shall terminate as of the date of occurrence of such damage. Nothing herein
shall obviate Lessee's obligations under Paragraph 8.

     9.2  Insurance Proceeds.  The proceeds of any insurance maintained under
          ------------------                                                 
Paragraph 8.3 hereof shall be paid to Lessor; however, such proceeds shall be
made available to Lessee for payment of costs and expense of repair; provided,
however, that such proceeds may be made available to Lessee subject to
reasonable conditions, including, but not limited to architect's certification
of cost, retention of a percentage of such proceeds pending recordation of a
notice of completion and a lien and completion bond to insure against mechanics'
or materialmen's liens (or compliance with such other applicable and comparable
legal and customary procedures within the State for insuring against any such
liens) arising out of the repair and to insure completion of the repair, all at
the expense of Lessee.

     9.3  Damage Near End of Term.
          ----------------------- 

          (a)  If the Premises are substantially damaged or destroyed, either
     partially or totally, during the last six (6) months of the term of this
     Lease or any extension term, Lessor or Lessee may at their option, cancel
     and terminate this Lease as of the date of occurrence of such damage by
     giving written notice to the other of their election to do so within ten
     (10) days after the date of occurrence of such damage; provided, that the
     parties shall not be permitted to exercise such election to cancel this
     Lease in the event such damage is not in excess of ten percent (10%) of the
     value of the Improvements and either (i) can be repaired within thirty (30)
     days, as determined by Landlord's independent, licensed architect, which
     determination shall be made as soon as reasonably possible and shall be
     conclusive on the parties or (ii) has in fact been repaired by Lessee at
     its sole cost and expense within thirty (30) days after the occurrence of
     the damage.

          (b)  Notwithstanding Paragraph 9.3(a) to the contrary, in the event
     that Lessee has an option to extend or renew this Lease, and the time
     within which said option may be exercised has not yet expired, Lessee shall
     exercise such option, if it is to be

                                     -18-
<PAGE>
 
     exercised at all, no later than twenty (20) days after damage or
     destruction to the Premises, either total or partial, occurring during the
     last six (6) months of the term of this Lease or any extension term, which
     damage or destruction is covered by insurance required to be maintained
     under Section 8. If Lessee duly exercises such option during said twenty
     (20) day period, Lessee shall, in accordance with Paragraph 9.2, at
     Lessee's expense, repair such damage as soon as reasonably possible and
     this Lease shall continue in full force and effect. If Lessee fails to
     exercise such option during said twenty (20) day period, then Lessor may,
     at Lessor's option, terminate and cancel this Lease as of the expiration of
     said twenty (20) day period by giving written notice to Lessee of Lessor's
     election to do so within ten (10) days after the expiration of said twenty
     (20) day period, notwithstanding any term or provision in the grant of
     Option to the contrary.

          (c)  Notwithstanding anything express or implied to the contrary in
     the foregoing, if Lessee fails to exercise its option to extend pursuant to
     Paragraph 9.3(b), then Lessee's presence on the Premises shall not be
     deemed a holdover during a period of ninety (90) days after giving notice,
     provided that Lessee uses its best efforts to move its equipment from the
     Premises as soon as is reasonably possible, Lessee's presence is not in
     violation of any laws, and Lessee complies with Lessor's reasonable safety
     and insurance requirements.

     9.4  No Abatement of Rent.  Notwithstanding the partial or total 
          --------------------                                       
destruction of the Premises or any part thereof, and notwithstanding whether the
casualty is insured or not, there shall be no abatement of rent or of any other
obligation of Lessee hereunder by reason of such damage or destruction unless
the Lease is terminated by virtue of any other provision of this Lease.

     9.5  Termination Advance Payments.   Upon termination of this Lease
          ----------------------------                                  
pursuant to this Section 9, an equitable adjustment shall be made concerning
advance rent and any advance payments made by Lessee to Lessor. Notwithstanding
any right of Lessor to terminate this Lease prior to the expiration hereof,
Lessee may elect, in writing, within ten (10) days of Lessor's notice of
termination, to continue to possess the undamaged portions of the Premises (to
the extent permitted by law), at the full rental prescribed herein and without
the abatement of rent.

                                     -19-
<PAGE>
 
     9.6  Waiver.  Lessee waives the provisions of Va. Code Ann. (S) 55-226 and
          ------                                                               
any other statutes hereinafter enacted which relate to the abatement of rent,
repair of premises and/or termination of leases when the premises leased are
destroyed and agrees that such event shall be governed by the terms of this
Lease.

                                  SECTION 10

                              REAL PROPERTY TAXES

     10.1 Payment of Taxes.  Lessee shall pay the real property tax, as defined
          ----------------                                                     
in Paragraph 10.2, applicable to the Premises (including the Excess Land as
defined in Section 47) during the term of this Lease; provided, however, that if
                                                      --------  ------- 
at any time there exists any Released Land (as defined in-Section 47) Lessor
shall pay to Lessee from time to time, within thirty (30) days after being
billed therefor by Lessee, an amount equal to the sum of (a) the product of any
real property tax previously paid by Lessee for any period during the term of
this Lease attributable to the ''Land" portion of the Premises, including the
Excess Land, multiplied by a fraction, the numerator of which shall be the
number of square feet of land in the Released Land and the denominator of which
shall be the number of square feet of land in the Premises, including the Excess
Land, and (b) the product of any real property tax previously paid by Lessee for
any period during the term of this Lease attributable to the "Improvements"
portion of the Premises, including the Excess Land, multiplied by a fraction,
the numerator of which shall be the number of square feet of interior space in
all buildings on the Released Land and the denominator of which shall be the
number of square feet of interior space in all buildings on the Premises,
including the Released Land, to the extent not previously paid by Lessor. All
such payments shall be made prior to the due date of such payment. Promptly
after payment thereof, Lessee shall furnish Lessor with copies of receipted tax
bills, to the extent received by Lessee, showing that such taxes have been
timely paid. In the event Lessor receives the real property tax bill directly,
Lessor shall make best efforts to notify Lessee of such obligation prior to a
penalty attaching but in no event more than ten (10) days after receipt. If any
such taxes paid by Lessee shall cover any period of time after the expiration of
the term hereof, Lessee's share of such taxes shall be equitably prorated to
cover only the period of time within the tax fiscal year during which this Lease
shall be in effect, and Lessor shall reimburse Lessee to the extent required. If
Lessee shall fail to pay any such taxes, Lessor shall have the right to pay the
same, in which

                                     -20-
<PAGE>
 
case Lessee shall repay such amount to Lessor with Lessee's next rent
installment together with interest at the lesser of (a) the maximum rate then
allowable by law or (b) 1.0% above the interest rate announced as its "prime"
rate by Sovran Bank, N.A. per annum, for the first thirty (30) days such taxes
remain unpaid and 4.0% above the interest rate announced as its "prime" rate by
Sovran Bank, N.A., per annum thereafter. Immediately upon the execution of this
Lease, Lessee shall request of all relevant taxing authorities that duplicate
tax bills be sent to Lessor at the address set forth by Lessor's signature.

     10.2  Definition of "Real Property Tax".  As used herein, "real property
           ----------------------------------                                
tax" shall include the following:

          (a)  any form of real estate tax or assessment, general, special,
     ordinary or extraordinary, and any license fee, commercial rental tax,
     improvement bond or bonds, levy or tax (other than inheritance, income or
     estate taxes) imposed on the Premises by any authority having the direct or
     indirect power to tax, including any county, state or federal government,
     or any school, agricultural, sanitary, fire, street, drainage or other
     improvement district thereof, as against any legal or equitable interest of
     Lessor in the Premises or in the real property of which the Premises are a
     part, as against Lessor's right to rent or other income therefrom, and as
     against Lessor's business of leasing the Premises, so long as such are not
     levied or assessed as substitutes or in lieu of taxes to be paid by Lessor
     hereunder;

          (b)  any assessment, tax, fee, levy or charge in substitution,
     partially or totally, of any assessment, tax, fee, levy or charge
     previously included within the definition of real estate tax, including
     assessments, taxes, fees, levies and charges which may be imposed by
     governmental agencies for such services as fire protection, street,
     sidewalk and road maintenance, refuse removal and for other governmental
     services formerly provided without charge to property owners or occupants.
     It is the intention of Lessee and Lessor that all such new or adjusted
     assessments, taxes, fees, levies and charges be included within the
     definition of "real property taxes" for the purposes of this Lease;

                                     -21-
<PAGE>
 
          (c)  Any assessment, tax, business, professional or occupational
     license tax, fee, levy or charge allocable to or measured by the area of
     the Premises or the rent payable hereunder, including, without limitation,
     any gross income tax or excise tax levied by the State, County or federal
     government, or any political subdivision thereof, with respect to the
     receipt of such rent, or upon or with respect to the possession, leasing,
     operating, management, maintenance, alteration, repair, use or occupancy by
     tenants of the Premises, or any portion thereof;

          (d)  any assessment, tax, fee, levy or charge upon this transaction or
     any document to which Lessee is a party, creating or transferring an
     interest or an estate in the Premises;

          (e)  any assessment, tax, fee, levy or charge by any governmental
     agency related to any transportation plan, fund or system instituted within
     the geographic area of which the Premises are a part;

          (f)  reasonable legal, consultants' and other professional fees, costs
     and disbursements incurred in connection with proceedings to contest,
     determine or reduce real property taxes;

          (g)  any rent taxes or gross receipt taxes (whether assessed against
     Lessee and collected by Lessor, or both);

          (h)  any levy, assessment or charge by any governmental authority
     against the Premises or Lessor or Lessee as a consequence of the occurrence
     of "Hazardous Materials" (as defined in Paragraph 45.2 below) in, on or
     about the Premises, or the leaching or migration thereof to other
     properties.

          (i)  any levy, assessment or charge against the Premises or Lessor or
     Lessee pursuant to any covenants, easements, agreements, reciprocal
     easement agreements or similar documents applicable to or affecting the
     Premises or Lessor or Lessee.

          It is the intent of the foregoing that such obligation shall include
only such taxes which would be assessed on or against the Premises even were the
Premises the only real property owned by Lessor, and shall not include taxes on
Lessor's net income, franchise, estate, succession or inheritance taxes or taxes
based upon capital levies.

                                     -22-
<PAGE>
 
          10.3 Personal Property Taxes.
               ----------------------- 

          (a)  Lessee shall pay prior to the date due all taxes assessed against
     and levied upon trade fixtures, furnishings, equipment and all other
     personal property of Lessee contained in the Premises. When possible,
     Lessee shall cause said trade fixtures, furnishings, equipment and all
     other personal property to be assessed and billed separately from the real
     property of Lessor.

          (b)  If any of Lessee's said personal property shall be assessed with
     Lessor's real property, Lessee shall pay Lessor the taxes attributable to
     Lessee within thirty (30) days after receipt of a written statement setting
     forth the taxes applicable to Lessee's property.

                                  SECTION 11

                                   UTILITIES

     Lessee shall pay for all water, gas, heat, light, power, telephone and
other utilities and services supplied to the Premises, together with any taxes
thereon. Lessor will attempt to provide separate metering but if any such
services are not separately metered to Lessee, Lessee shall pay a reasonable
proportion to be reasonably determined by Lessor of all charges jointly metered
with other premises.

                                  SECTION 12

                           ASSIGNING AND SUBLETTING

     12.1 Lessor's Consent Required.  Lessee shall not voluntarily or by 
          -------------------------                                     
operation of law assign, transfer, mortgage, sublet, or otherwise transfer or
encumber all or any part of Lessee's interest in this Lease or in the Premises,
without Lessor's prior written consent, which Lessor shall not unreasonably
withhold. Notwithstanding the immediately preceding sentence, Lessee may grant
the U.S. Government licenses to have U.S. Government employees operate out of
the Premises in connection with the performance by Lessee of Lessee contracts
with the U.S. Government, but such licenses shall automatically terminate as to
the Premises and any part thereof upon the termination of Lessee's right to
occupy the Premises or such part pursuant to this Lease. Lessor shall

                                    -23-
<PAGE>
 
respond to Lessee's request for consent hereunder within fifteen (15) business
day's after the Lessee shall have furnished Lessor with a copy of the proposed
sublease or assignment, reasonable detail regarding the identity of the proposed
sublessee or assignee and the business terms of the proposed sublease or
assignment and such other financial and other information as Lessor may request.
Any attempted assignment, transfer, mortgage, encumbrance or subletting without
such consent shall constitute a breach of this Lease.

     12.2 Lessee Affiliate.  Notwithstanding the provisions of Paragraph 12.1
          ----------------                                                   
hereof, Lessee may assign or sublet the Premises, or any portion thereof,
without Lessor's consent, to any corporation which controls, is controlled by or
is under common control with Lessee, or to any corporation resulting from the
merger or consolidation with Lessee, or to any person or entity which acquires
all the assets of Lessee as a going concern of the business that is being
conducted on the Premises, provided that said assignee assumes, in full, the
obligations of Lessee under this Lease and provided further that Lessee delivers
to Lessor written notice of such assignment within five (5) business days after
the occurrence thereof. Any such assignment shall not, in any way, affect or
limit the liability of Lessee under the terms of this Lease even if after such
assignment or subletting the terms of this Lease are materially changed or
altered without the consent of Lessee, the consent of whom shall not be
necessary.

     12.3 No Release of Lessee.    Regardless of Lessor's consent, no
          --------------------                                       
subletting or assignment shall release Lessee's obligation or alter the primary
liability of Lessee to pay the Rent and to perform all other obligations to be
performed by Lessee hereunder. The acceptance of Rent by Lessor from any other
person shall not be deemed to be a waiver by Lessor of any provision hereof!
Consent to one assignment or subletting shall not be deemed consent to any
subsequent assignment or subletting. In the event of default by any assignee of
Lessee or any successor of Lessee in the performance of any of the terms hereof,
Lessor may proceed directly against Lessee without the necessity of exhausting
remedies against said assignee. Lessor may consent to subsequent assignments or
subletting of this Lease or amendments or modifications to this Lease with
assignees of Lessee, with Lessee's prior written approval, which approval shall
not be unreasonably withheld or delayed. In the event Lessee does not respond to
Lessor in writing within ten (10) days after a written request for its consent
hereunder, Lessee conclusively shall be deemed to have consented. Any such
approved change or amendment shall not relieve Lessee of liability under this
Lease.

                                    -24-
<PAGE>
 
     12.4 Attorneys Fees.  In the event Lessee shall assign or sublet the
          --------------                                                 
Premises or request the consent of Lessor to any assignment or subletting, then
Lessee shall pay Lessor's actual attorneys' fees incurred in connection
therewith, such attorneys' fees not to exceed $1,000.00 for each such request.

                                  SECTION 13

                              DEFAULTS; REMEDIES

     13.1 Defaults.  The occurrence of any one or more of the following events
          --------                                                            
shall constitute a material default and breach of this Lease by Lessee:

          (a)  The vacating or abandonment of the Premises by Lessee; provided,
     however, that a vacation or abandonment of the Premises by Lessee shall not
     constitute a material default and breach of this Lease so long as Lessee
     complies with all of its obligations contained in this Lease and, in
     addition, provides, in a manner satisfactory to Lessor, for adequate
     security and other measures deemed necessary or desirable by Lessor for the
     maintenance of the Premises in good order, condition and repair due' to the
     vacation or abandonment of the Premises by Lessee.

          (b)  The failure by Lessee to make any payment of Rent or any other
     payment required to be made by Lessee hereunder, as and when due, where
     such failure shall continue for a period of ten (10) days after written
     notice thereof from Lessor to Lessee that such amount is past due. In the
     event that Lessor serves Lessee with a Notice to Pay Rent or Quit (or
     similar notice under applicable law) pursuant to applicable Unlawful
     Detainer statutes such Notice to Pay Rent or Quit shall also constitute the
     10-day notice required by this subparagraph. Notwithstanding anything
     express or implied to the contrary in the foregoing, to the extent Lessee
     has not received from Lessor notice of the recalculation of the Monthly
     Basic Rent as set forth in Paragraph 4.1 above, Lessee's payment of the
     Monthly Basic Rent due for the prior month shall not constitute a default
     hereunder, provided that Lessee pays any deficiency due within ten (10)
     days after receipt of the notice of recalculation of the Monthly Basic
     Rent.

                                     -25-
<PAGE>
 
          (c)  The failure by Lessee to observe or perform any of the covenants,
     conditions or provisions of this Lease to be observed or performed by
     Lessee, other than described in Paragraph (b) above, where such failure
     shall continue for a period of thirty (30) days after written notice hereof
     from Lessor to Lessee; provided, however, that if the nature of Lessee's
     default is such that more than thirty (30) days are reasonably required for
     its cure, then Lessee shall not be deemed to be in default if Lessee
     commenced such cure within said thirty (30) day period and thereafter
     diligently prosecutes such cure to completion.

          (d)  (i)  The making by Lessee of any general arrangement-or
     assignment for the benefit of creditors; (ii) Lessee becomes a "debtor" as
     defined in 11 U.S.C. Section 101 or any successor statute thereto (unless
     in the case of a petition filed against Lessee, the same is dismissed
     within one hundred twenty (120) days); (iii) the appointment of a trustee
     or receiver to take possession of substantially all of Lessee's assets
     located at the Premises or of Lessee's interest in this Lease, where
     possession is not restored to Lessee within one hundred twenty (120) days;
     or (iv) the attachment, execution or other judicial seizure of
     substantially all of Lessee's assets located at the Premises or of Lessee's
     interest in this Lease, where such seizure i not discharged within one
     hundred twenty (120) days. However, in the event that any provision of this
     Paragraph 13.1(d) is contrary to any applicable law, such provision shall
     be of no force or effect.

          (e)  The discovery by Lessor that any financial statement given to
     Lessor by Lessee, any assignee of Lessee, any subtenant of Lessee, any
     successor in interest of Lessee or any guarantor of Lessee's obligation
     hereunder, and any of them, was materially false.

          (f)  The violation of Section 411 of the Employee Retirement Income
     Security Act of 1974 ("ERISA") by Lessee or the causing of such a violation
     by Lessee.

          (g)  The breach of the representations or covenants contained in
     Section 45, the appearance, handling, use, presence, transport or disposal
     of any

                                     -26-
<PAGE>
 
     "Hazardous Material" (as defined in Paragraph 45.2 below) on or about the
     Premises not in compliance with all "Governmental Regulations" (as defined
     in Paragraph 45.3 below) applicable thereto, and the failure of Lessee to
     diligently and faithfully pursue and keep in place the clean-up and/or
     remedial measures which may be required from time-to-time by any
     governmental authority.

     13.2 Remedies.  Upon occurrence of a material default, Lessor shall have
          --------                                                           
the following remedies, in addition to all other rights and remedies provided by
law or otherwise provided in this Lease, to which Lessor may resort cumulatively
or in the alternative:

          (a)  Lessor may continue this Lease in full force and effect, and this
     Lease shall continue in full force and effect as long as Lessor does not
     terminate this Lease, and Lessor shall have the right to collect Monthly
     Basic Rent when due;

          (b)  Lessor may terminate Lessee's right to possession of the Premises
     at any time by giving written notice to that effect, and relet the Premises
     or any part thereof. Lessee shall be liable immediately to Lessor for all
     reasonable costs Lessor incurs in reletting the premises or any part
     thereof, including, without limitation, broker's commissions, expenses of
     cleaning, redecorating, and preparing the Premises to be re-let and like
     costs. Reletting may be for a period shorter or longer than the remaining
     term of this Lease. No act by Lessor other than giving written notice to
     Lessee shall terminate this Lease. Acts of maintenance, efforts to relet
     the Premises or the appointment of a receiver on Lessor's initiative to
     protect Lessor's interest under this Lease shall not constitute a
     termination of Lessee. right to possession. Upon termination, Lessor shall
     have the right to remove all of Lessee's personal property and store same
     at Lessee's cost and to recover from Lessee as damages:

               (1)  The worth at the time of award or any unpaid Monthly Basic
          Rent and other sums due and payable which had been earned at the time
          of termination; plus

               (2)  The worth at the time of award Ct the amount by which the
          unpaid Rent and other

                                    -27-
<PAGE>
 
               sums which would have been payable after termination until the
               time of award exceeds the amount of such Rent loss that Lessee
               proves could have been reasonably avoided; plus

                    (3) The worth at the time of award of the amount by which
               the unpaid Rent and other sums due for the balance of the term of
               this Lease after the time of award exceeds the amount of such
               Rent loss that Lessee proves could be reasonably avoided; plus

                    (4) Any other amounts reasonably necessary to compensate
               Lessor for all the detriment proximately caused by Lessee's
               failure to perform Lessee's obligations under this Lease, or
               which, in the ordinary course of things, would be likely to
               result therefrom, including, without limitation, any costs or
               expenses incurred by Lessor: (i) in retaking possession of the
               premises; (ii) in maintaining, repairing, preserving, restoring,
               replacing, cleaning, altering or rehabilitating the Premises or
               any portion thereof, including such acts for reletting to a new
               lessee or lessees; (iii) for leasing commissions; or (iv) for any
               other costs necessary or appropriate to relet the Premises; plus

                    (5) At Lessor's election, such other amounts and remedies in
               addition to or in lieu of the foregoing as may be permitted from
               time to time by the laws of the State including, without
               limitation, the remedies provided by Chapter 13 of Title 55 of
               the Code of Virginia, Ann., as amended from time to time.

                    The "worth at the time of award" of the amounts referred to
          in Paragraphs 13.2(b)(1) and 13.2(b)(2) is computed by allowing
          interest at the prime rate promulgated by Sovran Bank, N.A. plus one
          percent (1%) ("Interest Rate") on the unpaid Monthly Basic Rent and
          other sums due and payable from the termination date through the date
          of award. The "worth at the time of award" of the amount referred to
          in Paragraph 13.2(b) (3) is computed by discounting such amount at the
          discount rate of the Federal Reserve Bank of Richmond at the time of
          award plus one percent (1%). Lessee waives redemption or relief from

                                     -28-
<PAGE>
 
          forfeiture under any present or future law, in the event Lessee is
          evicted or Lessor takes possession of the Premises by reason of any
          default of Lessee hereunder; or

               (c) Lessor may, with or without terminating this Lease, re-enter
          the Premises and remove all persons and property from the Premises;
          such property may be removed and stored in a public warehouse or
          elsewhere at the cost of and for the account of Lessee. No re-entry or
          taking possession of the Premises by Lessor pursuant to this
          Subparagraph shall be construed as an election to terminate this Lease
          unless a written notice of such intention is given to Lessee.

          13.3  Default By Lessor. Lessor shall not be in default hereunder
                -----------------   
unless Lessor fails to perform obligations required of Lessor in this Lease
within a reasonable time, but in no event later than thirty (30) days after
written notice by Lessee to Lessor and to the holder of any first mortgage or
deed of trust covering the Premises whose name and address shall have
theretofore been furnished to Lessee in writing specifying wherein Lessor has
failed to perform such obligation; provided, however, that if the nature of
Lessor's obligation is such that more than thirty (30) days are required for
performance then Lessor shall not be in default if Lessor commences performance
within such thirty (30) day period and thereafter diligently prosecutes the same
to completion.

          13.4  Late Charges. Lessee hereby acknowledges that late payment by
                ------------
Lessee to Lessor of rent and other sums due hereunder will cause Lessor to incur
costs not contemplated by this Lease, the exact amount of which will be
extremely difficult to ascertain. Such costs include, but are not limited to,
processing and accounting charges, and late charges which may be imposed on
Lessor by the terms of any mortgage or trust deed covering the Premises.
Accordingly, beginning with the third and all subsequent occurrences of any
installment of rent or any other sum due from Lessee not being received by
Lessor or Lessor's designee within ten (10) days after such amount shall be due,
then, without any requirement for notice to Lessee, Lessee shall pay to Lessor,
beginning with such third late payment, a late charge within ten (10) days after
each such amount shall be due, equal to four percent (4%) of such overdue
amount. The parties hereby agree that such late charge represents a fair and
reasonable estimate of the costs Lessor will incur by reason of late payment by
Lessee. Acceptance of such late charge by Lessor shall in no event constitute a
waiver 

                                     -29-
<PAGE>
 
of Lessee's default with respect to such overdue amount, nor prevent
Lessor from exercising any of the other rights and remedies granted hereunder.

          13.5  Impounds.  In the event that a late charge is payable hereunder,
                --------                                                        
whether or not collected, for three (3) installments of rent or any other
monetary obligation of Lessee under the terms of this Lease during any twelve
(12) month period during the term hereof, Lessee shall pay to Lessor, if Lessor
shall so request, in addition to any other payments required under this Lease, a
monthly advance installment, payable at the first of each month, as estimated by
Lessor, for real property tax and insurance expenses attributable to the
Premises which are payable by Lessee under the terms of this Lease. Such fund
shall be established to insure payment when due, before delinquency, of any or
all such real property taxes and insurance premiums. If the amounts paid to
Lessor by Lessee under the provisions of this paragraph are insufficient to
discharge the obligations of Lessee to pay such real property taxes and
insurance premiums as the same become due, Lessee shall pay to Lessor, upon
Lessor's demand, such additional sums necessary to pay such obligations. All
moneys paid to Lessor under this paragraph may be intermingled with other moneys
of Lessor and shall not bear interest. In the event of a default in the
obligations of Lessee to perform under this Lease, then any balance remaining
from funds paid to Lessor under the provisions of this paragraph may, at the
option of Lessor, be applied to the payment of any monetary default of Lessee in
lieu of being applied to the payment of real property tax and insurance
premiums. After satisfaction of all monetary obligations of Lessee, Lessor shall
refund to Lessee any funds collected hereunder that remain unspent at the
expiration of Lease herein.

                                  SECTION 14

                                 CONDEMNATION

          If the Premises or any portion thereof are taken under the power of
eminent domain, or sold under the threat of the exercise of said power (all of
which are herein called "condemnation"), this Lease shall terminate as to the
part so taken as of the date the condemning authority takes title or possession,
whichever first occurs. If more than fifteen percent (15%) of the floor area of
either building or the Premises, or more than fifteen percent (15%) of the
available parking for the Premises such that minimum legal parking for the
Premises can no longer be provided, is taken by condemnation,

                                     -30-
<PAGE>
 
Lessee may, at Lessee's option, to be exercised in writing only within ten (10)
days after Lessor shall have given Lessee (or in the absence of such notice,
condemning authority shall have taken possession) terminate this Lease as of the
date the condemning authority takes such possession (unless, in the case of
condemnation of more than fifteen percent (15%) of the parking for the Premises,
the remaining parking can be re-striped by Lessor to provide the necessary
parking to meet applicable zoning requirements or Lessor otherwise makes Lessee
improved parking spaces in the same number exist, at such locations as are
reasonably Lessee). with the available to as currently acceptable to Lessee). If
Lessee does not terminate this Lease in accordance with the foregoing, this
Lease shall remain in full force and effect as to the portion of the Premises
remaining, except that the rent shall be reduced in the proportion that the
floor area of the building-taken bears to the total floor area of the building
situated on the Premises. Lessee's rent shall be abated in the event that a
portion of the unimproved area of the Premises is taken, based upon an equitable
reduction in rent, as reasonably agreed upon in good faith by Lessor and Lessee,
in accordance with the same procedures used at the commencement of this Lease
for calculating fair market rental to reflect the decreased value of the
Premises (and, therefore, the fair market rental) by virtue of such taking;
provided, however, such calculation shall not result in a fair market value for
the Premises that is less than the fair market value that would be calculated by
subtracting the amount of the cash award (less all costs and expenses incurred
by Lessor in connection with such condemnation) awarded to Lessor as a result of
such condemnation from the fair market value of the Premises prior to such
condemnation. Any award for the taking of all or any part of the Premises under
the power of eminent domain or any payment made under threat of the exercise of
such power shall be the property of Lessor, whether such award shall be made as
compensation for diminution in value of the leasehold or for the taking of the
fee, or as severance damages; provided, however, that Lessee shall be entitled
to any award for loss of or damage to the Special Improvements and Lessee's
trade fixtures and removable personal property and, to the extent specifically
provided for in the award, moving expenses. In the event that this Lease is not
terminated by reason of such condemnation, Lessor shall to the extent of
severance damages received by Lessor in connection with such condemnation repair
any damage to the Premises caused by such condemnation except to the extent that
Lessee has been reimbursed therefor by the condemning authority. Lessee and
Lessor each shall pay one-half (1/2) of any amount in excess of such severance
damages required to complete such repair provided  

                                     -31-
<PAGE>
 
however, if the condemnation award is made during the last year of the term
hereof, or the last year of any extension term then such excess shall be the
responsibility of Lessor.

                                  SECTION 15

                              ESTOPPEL STATEMENT

          15.1 Lessee Estoppel. Lessee shall at any time upon not less than ten
               ---------------   
(10) days' prior written notice from Lessor execute, acknowledge and deliver to
Lessor a statement in writing (i) certifying that this Lease is unmodified and
in full force and effect (or, if modified, stating the nature of such
modification and certifying that this Lease, as so modified, is in full force
and effect) and the date to which the rent and other charges are paid in
advance, if any (ii)--that Lessee has no right of first refusal or other option
to purchase the Premises except pursuant to Paragraph 5 hereof (and confirming
that Lessee has waived such right, if such is the case), and (iii) acknowledging
that there are not, to Lessee's knowledge, any uncured defaults on the part of
Lessor hereunder, or specifying such defaults if any are claimed. Any such
statement may be conclusively relied upon by any prospective purchaser or
encumbrancer of the Premises.

          15.2 Failure to Deliver Lessee Estoppel. Without limiting Lessor's
               ----------------------------------    
remedies, Lessee's failure to deliver such statement within such time shall, at
Lessor's option, be conclusive upon Lessee (i) that this Lease is in full force
and effect, without modification except as may be represented by Lessor, (ii)
that there are no uncured defaults in Lessor's performance, and (iii) that not
more than one month's rent has been paid in advance.

          15.3 Financial Information. If Lessor desires to finance, refinance,
               --------------------- 
or sell the Premises, or any part thereof. Lessee hereby agrees to deliver to
any lender or purchaser designated by Lessor such financial statements of Lessee
as Lessee shall have prepared in the ordinary course and as may be reasonably
required by such lender or purchaser. Such statements shall include the past
three years' financial statements of Lessee. All such financial statements shall
be received by Lessor and such lender or purchaser in confidence and shall be
used only for the purposes herein set forth.

          15.4 Lessor Estoppel. Lessor shall-provide to Lessee from time to time
               --------------- 
upon not less than ten (10) days prior 

                                     -32-
<PAGE>
 
notice from Lessee, an estoppel statement in the form described in Paragraph
15.1 (but not subject to 15.2).

                                  SECTION 16

                              LESSOR'S LIABILITY

          Lessor and Lessee agree that this is a "Triple-Net" lease and that all
costs, taxes, repairs, expenses capital or otherwise incurred by or imposed on
the Premises or the Ownership thereof shall be paid by and be the responsibility
of Lessee. The term "Lessor" as used herein shall mean only the owner or owners,
at the time in question, of the fee title to the Premises. In the event of any
transfer of such title or interest, the Lessor herein named (and in case of any
subsequent transfers, the then current Lessor) shall be relieved from and after
the date of such transfer of all liability as respects Lessor's obligations
thereafter to be performed, provided that any funds in the hands of Lessor or
the then current Lessor at the time of such transfer, in which Lessee has an
interest, shall be delivered to the Lessor to whom the assignment is being made.
The obligations contained in this Lease to be performed by Lessor shall, subject
to the foregoing, be binding on Lessor's successors and assigns, only during
their respective periods of ownership.

                                  SECTION 17

                                 SEVERABILITY

          The invalidity of any provision of this Lease as determined by a court
of competent jurisdiction shall in no way effect the validity of any other
provision hereof.

                                  SECTION 18

                       INTEREST ON PAST-DUE OBLIGATIONS

          Except as expressly herein provided, any amount due to Lessor not paid
when due or within the applicable cure period in Paragraph 13.1 shall bear
interest at the lesser of the prime rate promulgated by Sovran Bank, N.A. plus
one percent (1%) or the maximum rate then allowable by law from the date due
(the "Interest Rate''). Payment of such interest shall not excuse or cure any
default by Lessee under this Lease provided, however,  

                                     -33-
<PAGE>
 
that interest shall not be payable on late charges incurred by Lessee.

                                  SECTION 19

                                TIME OF ESSENCE

          Time is of the essence.

                                  SECTION 20

                                ADDITIONAL RENT

          Any monetary obligations of Lessee to Lessor under the terms of this
Lease shall be deemed to be Rent.

                                  SECTION 21

                 INCORPORATION OF PRIOR AGREEMENTS; AMENDMENTS

          This Lease contains all agreements of the parties with respect to any
matter mentioned herein other than the Purchase Agreement. No prior agreement or
understanding pertaining to any such matter shall be effective, other than the
Purchase Agreement. This Lease may be modified in writing only, signed by the
parties in interest at the time of the modification. Except as otherwise stated
in this Lease, Lessee hereby acknowledges that neither the Lessor nor any
employees or agents has made any oral or written warranties or representations
to Lessee relative to the condition or use by Lessee of said Premises and Lessee
acknowledges that Lessee assumes all responsibility regarding the Occupational
Safety Health Act, as may be amended, and all other laws covering similar
matters, the legal use and adaptability of the Premises and the compliance
thereof with all applicable laws and regulations in effect during the term of
this Lease except as otherwise specifically stated in this Lease. This Section
is not meant to supersede any of the obligations or rights of Lessee and Lessor
contained in or pursuant to the Purchase Agreement. 

                                     -34-
<PAGE>
 
                                  SECTION 22

                                    NOTICES

          Any notice required or permitted to be given hereunder shall be in
writing and may be given by personal delivery or by certified mail and, if given
personally or by mail, shall be deemed sufficiently given if addressed to Lessee
or to Lessor at the address noted below the signature of the respective parties,
as the case may be. Either party may by notice to the other specify a different
address for notice purposes. A copy of all notices required or permitted to be
given hereunder shall be concurrently transmitted to such party or parties at
such addresses as Lessor or Lessee may from time to time hereafter designate by
notice to the other.

                                  SECTION 23

                                    WAIVERS

          No waiver by Lessor or any provision hereof shall be deemed a waiver
of any other provision hereof or of any subsequent breach by Lessee of the same
or any other provision. Lessor's consent to, or approval of, any act shall not
be deemed to render unnecessary the obtaining of Lessor's consent: to or
approval of any subsequent act by Lessee. The acceptance of rent hereunder by
Lessor shall not be a waiver of any preceding breach by Lessee of any provision
hereof, other than the failure of Lessee to pay the particular rent so accepted,
regardless of Lessor's knowledge of such preceding breach at the time of
acceptance of such rent.

                                  SECTION 24

                                 HOLDING OVER

          If Lessee, with Lessor's consent, remains in possession of the
Premises or any part thereof after the expiration of the term hereof, such
occupancy shall be a tenancy at sufferance upon all the provisions of this Lease
pertaining to the obligations of Lessee, but all options and rights of first
refusal, if any, granted under the terms of this Lease shall be deemed
terminated and be of no further effect during said tenancy, and Lessee shall pay
rent on a monthly basis in an amount equal to one hundred fifty percent (150%)
of the most recently due Monthly Basic Rent as adjusted pursuant to Paragraph
4.2, plus an amount of all additional rent which would 

                                     -35-
<PAGE>
 
be required to be paid hereunder. Further, Lessee shall also be liable to Lessor
for all damages arising as a result of such "holding over", including
compensation for the loss of prospective tenants.

                                  SECTION 25

                              CUMULATIVE REMEDIES

          No remedy or election hereunder shall be deemed exclusive but shall,
wherever possible, be cumulative with all other remedies at law or in equity.

                                  SECTION 26

                           COVENANTS AND CONDITIONS

          Each provision of this Lease performable by Lessee shall be deemed
both a covenant and a condition.

                                  SECTION 27

                         BINDING EFFECT; CHOICE OF LAW

          Subject to any provisions hereof restricting assignment or subletting
by Lessee and subject to the provisions of Section 16, this Lease shall bind the
parties, their personal representatives, successors and assigns. This Lease
shall be governed by the laws of the State wherein the Premises are located.

                                  SECTION 28

                                 SUBORDINATION

          28.1  Generally. This Lease, at Lessor's option, may be subordinated
                ---------
to any ground lease, mortgage, deed of trust, or any other hypothecation or
security interest now or hereafter placed upon the real property of which the
Premises are a part and to any and all advances made on the security thereof and
to all renewals, modifications, consolidations, replacements and extensions
thereof. Notwithstanding such subordination, Lessee's right to quiet possession
of the Premises shall not be disturbed (including Lessee's Options under Section
5 and 43 hereof) if Lessee is not in default and so long as Lessee shall 

                                     -36-
<PAGE>
 
pay the rent and observe and perform all of the provisions of this Lease, unless
this Lease is otherwise terminated pursuant to its terms. If any mortgagee,
trustee or ground lessor shall elect to have this Lease prior to the lien of its
mortgage, deed of trust or ground lease, and shall give written notice thereof
to Lessee, this Lease shall be deemed prior to such mortgage, deed of trust, or
ground lease, whether this Lease is dated prior or subsequent to the date of
said mortgage, deed of trust or ground lease or the date of recording thereof.

          28.2 Execution of Documents.  Lessee agrees to execute any documents
               ----------------------                                         
required to effectuate any attornment, a subordination or to make this Lease
prior to the lien of any mortgage, deed of trust or ground lease, as the case
may be (subject to the non-disturbance provisions above). Upon Lessee's failure
to execute such documents within fifteen (15) days after Lessor's second written
demand, specifying that Lessee has failed to respond to Lessor's first written
demand, without limiting Lessor's other remedies, Lessor shall have the option
to execute such documents on behalf of Lessee as Lessee's attorney-in-fact.
Lessee does hereby make, constitute and irrevocably appoint Lessor as Lessee's
attorney-in-fact and in Lessee's name, place and stead, to execute such
documents in accordance with this Paragraph 28.2, provided however that Lessor
shall not exercise such right unless Lessor shall first have given Lessee no
less that thirty (30) days notice and so long as Lessor shall not thereafter
have received notice of Lessee's objection to Lessor's exercise of such right.

                                  SECTION 29

                                ATTORNEY'S FEES

          If either party named herein brings an action to enforce the terms
hereof or declare rights hereunder, the prevailing party in any such action, on
trial or appeal, shall be entitled to its actual fees and costs for attorneys
and other consultants.

                                  SECTION 30

                                LESSOR'S ACCESS

          Subject to compliance with then applicable governmental security
requirements and Lessee's reasonable rules and regulations with respect to
classified areas, Lessor and Lessor's agents shall have the right to enter the
Premises at 

                                     -37-
<PAGE>
 
reasonable times after reasonable notice for the purposes of inspecting the
same, showing the same to prospective purchasers, lenders, or lessees, and
making such alterations, repairs, improvements or additions to the Premises or
to the building of which they are a part as Lessor may deem necessary or
desirable (but without any obligation to do so) or which Lessee is required to
perform under this Lease but Lessee has failed to perform, pursuant to the
provisions of Paragraph 7.3 above, or for purposes of posting notices of non-
responsibility. Subject to the same such limitations, Lessor may at any time
place on or about the Premises any ordinary "For Sale" signs and Lessor may at
any time during-the last one hundred twenty (120) days of the term hereof place
on or about the Premises any ordinary "For Lease" signs, all without rebate of
rent or liability to Lessee.

                                  SECTION 31

                                   AUCTIONS

          Lessee shall not conduct, nor permit to be conducted either
voluntarily or involuntarily, any public auction upon the Premises without first
having obtained Lessor's prior written consent.

                                  SECTION 32

                                     SIGNS

          Lessee shall not place any additional sign upon the Premises without
Lessor's prior written consent; provided, however, that with respect to the
signs of Lessee (but not with respect to the signs of Lessee's successors or
assigns) Lessor shall not be entitled to withhold its consent to the placement
of Lessee's customary and usual signs and logos used by Lessee generally when it
occupies office and industrial buildings. All signs placed by Lessee upon the
Premises shall comply with applicable laws and regulations.

                                  SECTION 33

                                    MERGER

          The voluntary or other surrender of this Lease by Lessee, or a mutual
cancellation thereof, or a termination by Lessor, shall not work a merger, and
shall, at the option of Lessor, terminate all or any existing subtenancies or
may, at 

                                     -38-
<PAGE>
 
the option of Lessor, operate as an assignment to Lessor of any or all of such
subtenancies.

                                  SECTION 34

                                   GUARANTOR

          In the event that there is a guarantor of this Lease, said guarantor
shall have the same obligations as Lessee under this Lease.

                                  SECTION 35

                               QUIET POSSESSION

          Upon Lessee paying the rent for the Premises and observing and
performing all of the covenants, conditions and provisions on Lessee's part to
be observed and performed hereunder, Lessee shall have quiet possession of the
Premises for the entire term hereof subject to all of the provisions of this
Lease. The individuals executing this Lease on behalf or Lessor represent and
warrant to Lessee that they are fully authorized and legally capable of
executing this Lease on behalf of Lessor and that such execution is binding upon
all parties holding an ownership interest in the Premises.

                                  SECTION 36

                           MULTIPLE TENANT BUILDING

          In the event that the Premises are part of a larger building or group
of buildings, then Lessee agrees that it will abide by, keep and observe all
reasonable rules and regulations which Lessor may make from time to time for the
management, safety, care and cleanliness of the buildings and grounds, the
parking of vehicles and the preservation of good order therein as well as for
the convenience of other occupants and tenants of the buildings. The violations
of any such rules and regulations shall be deemed a material breach of this
Lease by Lessee.

                                     -39-
<PAGE>
 
                                  SECTION 37

                               SECURITY MEASURES

          Lessee hereby acknowledges that the Rent payable to Lessor hereunder
does not include the cost of guard service or other security measures, and that
Lessor shall have no obligation whatsoever to provide same. Lessee assumes all
responsibility for the protection of Lessee, its agents and invitees from acts
of third parties.

                                  SECTION 38

                                   EASEMENTS

          Subject to Lessee's rights hereunder, Lessor reserves to itself the
right, from time to time, to grant such easements, rights and dedications, for
ingress, egress, utilities or other purposes consistent with the use of the
Premises or as an accommodation to adjacent landowners, that Lessor deems
necessary or desirable, and to cause the recordation of Parcel Maps and
restrictions, as long as such easements, rights, dedications, maps and
restrictions do not unreasonably interfere with the use of the Premises by
Lessee or any Addition constructed by Lessee. Lessee shall sign any of the
aforementioned documents upon request of Lessor.

                                  SECTION 39

                           PERFORMANCE UNDER PROTEST

          If at any time a dispute shall arise as to any amount or sum of money
to be paid by one party to the other under the' provisions hereof, the party
against whom the obligation to pay the money is asserted shall have the right to
make payment "under protest" and such payment shall not be regarded as a
voluntary payment, and there shall survive the right on the part of said party
to institute suit for recovery of such sum. If it shall be adjudicated that
there was no legal obligation on the part of said party to pay such sum or any
part thereof, said party shall be entitled to recover such sum, or so much
thereof as it was not legally required to pay under the provisions of this Lease
plus interest thereon at a rate per annum equal to the Interest Rate.

                                     -40-
<PAGE>
 
                                  SECTION 40

                                   AUTHORITY

          Each individual executing this Lease on behalf of such entity
represents and warrants that he or she is duly authorized to execute and deliver
this Lease on behalf of said entity.

                                  SECTION 41

                                   CONFLICT

          Any conflict between the typewritten and handwritten provisions
(initialed by the parties) shall be controlled by the handwritten provisions.

                                  SECTION 42

                                    OPTIONS

          42.1  Definition. As used in this Section, the word "Options" includes
                ----------  
all of the following: (1) the right or option to extend the term of this Lease
or to renew this Lease; (2) any option or right of first refusal to lease the
Premises; (3) the right of first refusal granted pursuant to Section 5 hereof;
(4) the option or right to purchase and leaseback an Addition as described in
Paragraph 43.1(a) hereof; or (5) the right to develop excess land as described
in Section 47 hereof.

          42.2  Assignment.  Each Option granted to Lessee in this Lease may be
                ----------                                                     
assigned in conjunction with the assignment of Lessee's rights and obligations
under this Lease in accordance with Section 12 above.

          42.3  Multiple Options. Notwithstanding anything contained in this
                ---------------- 
Lease to the contrary, in the event that Lessee has any multiple Options
pursuant to this Lease, a later Option (of the same type) cannot be exercised
unless the prior Option has been duly exercised.

          42.4  Effect of Default on Options.
                ---------------------------- 

                (a) Lessee shall have no right to exercise an Option,
          notwithstanding any provision in the grant of Option to the contrary,
          (i) during the time commencing from a default pursuant to Paragraph
          13.1(b) or 13.1(c) and continuing until the default alleged in

                                     -41-
<PAGE>
 
                                  SECTION 43

                         OPTIONS OF LESSOR AND LESSEE

          43.1 Addition Purchase Option.
               ------------------------ 

                                     -42-
<PAGE>
 


                          [Page intentionally blank]


                                     




                                     -43-
<PAGE>
 
          43.2  Lessee's Lease Extension Options. Lessor hereby gives and grants
                -------------------------------- 
to Lessee the exclusive right and option to extend the term of this Lease for
four (4) consecutive five (5) year extension terms. Each such extension option
may only be exercised by written notice declaring Lessee's election to exercise
the extension option: given to Lessor not less than one-hundred eighty (180)
days prior to the expiration of the initial term or the then current extension
term preceding such extension term. All of the terms, covenants and conditions
of this Lease shall apply during such extension term, except that no more than
four (4) consecutive five (5) year extension terms shall be permitted (e.g. a
total possible extension of twenty (20) years), and except that the rent for
each renewal term shall be determined as follows:

                (a)  Promptly following the exercise of the extension option,
          the monthly fair market rental value of the Premises and Addition(s),
          if any, shall be 

                                     -44-
<PAGE>
 
          determined. Lessor and Lessee hereby agree that such monthly fair
          market rental value shall be the Monthly Basic Rent payable for the
          first month of the extension term, subject to adjustment pursuant to
          Paragraph 4.2 hereof. However, such fair market rental value for the
          first month of the extension term shall exclude any Addition which is
          not purchased by the Lessor, as described in Paragraph 43.1 herein.

                (b)  The adjustments to Monthly Basic Rent shall be in the same
          manner and at the same times as set forth in Paragraph 4.2 hereof.
          Notwithstanding the foregoing, in no event shall the rental payable
          for any month during an extension term be less than the rental payable
          for the first month of such extension term or the last month of the
          prior initial or extension term, as the case may be.

          43.3  Qualifications of Options. Lessor and Lessee recognize and agree
                -------------------------
herein that neither party during the last one hundred eighty (180) days of the
Lease term described in Paragraph 3 herein or the then current extension term
shall have option rights or privileges pursuant to this Section 43; provided,
that such limitation shall not apply to subsequent extension options granted
herein-where Lessee has timely exercised each prior extension option. If at the
expiration of the Lease term, neither party has exercised its option as
described herein, Lessor and Lessee shall be released of any and all
responsibilities and obligations under this Section 43.


                                  SECTION 44

                              DETERMINATION AND 
                           FAIR MARKET RENTAL VALUE

          44.1  Fair Market Rental Value. In each instance in this Lease where
                ------------------------
the fair market rental value of the Premises is to be determined, such
determination shall be made in accordance with the procedures set forth in this
Section 44. The fair market rental value shall be based on a triple-net lease
with tenants of the same quality as Lessee for comparable space in the Reston
area of Fairfax County including market concessions, if any.

          44.2  Arbitration Procedures. Promptly following the giving of notice
                ---------------------- 
or other occurrence giving rise to the need to determine fair market rental
value, the parties shall endeavor in good faith to determine such value. If
Lessor and Lessee fail to reach agreement as to fair market rental value by the

                                     -45-
<PAGE>
 
date twenty (20) days from the date of the giving of notice or other occurrence
giving rise to the need to determine value ("Specified Date"), such
determination shall be submitted to arbitration as follows:

               (a)  Lessor and Lessee shall each appoint one arbitrator who
          shall by profession be a real estate appraiser who shall have been
          active over the five (5) year period ending on the date of such
          appointment in the appraisal of commercial properties in the Northern
          Virginia area. The determination of the arbitrators shall be limited
          solely to the issue of whether Lessor's or Lessee's submitted figure
          for the fair market rental value is closest to the actual value as
          determined by the arbitrators, taking into account the requirements of
          this Section 44. Each such arbitrator shall be appointed within
          fifteen (15) days after the Specified Date.

               (b)  The two arbitrators so appointed shall within fifteen (15)
          days of the date of the appointment of the last appointed arbitrator
          agree upon and appoint a third arbitrator who shall be qualified under
          the same criteria set forth herein above for qualification of the
          initial two arbitrators.

               (c)  The three arbitrators shall within thirty (30) days of the
          appointment of the third arbitrator reach a decision as to whether the
          parties shall use Lessor's or Lessee' submitted values and shall
          notify Lessor and Lessee thereof.

               (d)  The decision of the majority of the three arbitrators shall
          be binding upon Lessor and Lessee.

               (e)  If either Lessor or Lessee fails to appoint an arbitrator
          within the time period specified herein above, the arbitrator
          appointed by one of them shall reach a decision, notify Lessor and
          Lessee thereof, and such arbitrator's decision shall be binding upon
          Lessor.

               (f)  If the two arbitrators fail to agree upon and appoint a
          third arbitrator, both arbitrators shall be dismissed and the matter
          to be decided shall be forthwith submitted to arbitration under the
          provisions of the American Arbitration Association.

                                     -46-
<PAGE>
 
               (g)  The cost of arbitration shall be paid by Lessor and Lessee
          equally.


                                  SECTION 45

                              HAZARDOUS MATERIALS

          45.1  Covenants of Lessee. Lessor and Lessee agree as follows with
                -------------------
respect to the existence or use of "Hazardous Material" (as defined in Paragraph
45.2 below) on the premises:

               (a)  Lessee hereby makes the following representations and
          covenants to Lessor:

                    (i)   Any handling, transportation, storage, treatment or
               use of Hazardous Material that has occurred on the Premises prior
               to the Commencement Date by Lessee, its employees, agents or
               tenants has been in compliance with all Applicable "Governmental
               Regulations" (as defined in Paragraph 45.3 below).

                    (ii)  Subject to Paragraph 45.1(a)(iii) below, the Premises
               are, as of the Commencement Date, in compliance with all
               Governmental Regulations regulating the handling, transportation,
               storage, treatment, use and disposition of Hazardous Material.

                    (iii) To the best of Lessee's knowledge, the soil and ground
               water on or under the Premises is substantially free of Hazardous
               Material.

               (b)  Lessee shall be responsible for all costs incurred in
          complying with all Governmental Regulations which relate to Hazardous
          Material on, in or about the Premises. Upon Lessor's reasonable
          determination that any such costs must be incurred, Lessee shall
          obtain for the benefit of Lessor a performance bond in an amount of
          such costs, as reasonably determined by Lessor, which Lessor may
          enforce in the event Lessee fails to comply with all remedial or 
          clean-up requirements of such Governmental Regulations.

               (c)  Lessee shall indemnify, defend and hold Lessor harmless from
          and against any and all claims,

                                     -47-
<PAGE>
 
          judgments, damages, penalties, fines, costs, liabilities or losses
          (including, without limitation, diminution in value of the Premises,
          damages for the loss or restriction on use of rentable or usable space
          or of any amenity of the Premises, damages arising from any adverse
          impact on the redevelopment of the Premises or the marketing of space
          therein, increased costs of demolition and removal of the improvements
          which are part of the Premises at the expiration or earlier
          termination of the Lease term, and sums paid in settlement of claims,
          attorneys' fees, consultant fees and expert fees) which arise during
          or after the Lease term from or in connection with Lessee's failure to
          maintain the levels of any Hazardous Materials present on and/or under
          the Premises below the levels established by applicable Governmental
          Regulations. Lessee agrees that its indemnification of Lessor
          includes, without limitation, costs incurred in connection with (i)
          the removal of the asbestos, if any, in the roofing materials of the
          Premises (provided that (A) Lessor is required by Governmental
          Regulation to remove such asbestos or (B) Lessor removes same if it
          reasonably believes it constitutes a health hazard, in either case
          occurring prior to the expiration or sooner termination of this
          Lease); (ii) any liability for personal injury or other liability
          arising during the term of the Lease in whole or in part due to the
          presence of asbestos in the roofing materials; and (iii) any
          investigation of site conditions or any clean-up, remedial, removal or
          restoration work required by any federal, state or local governmental
          agency or political subdivision because of Hazardous Materials present
          in the soil or ground water on, in or under the Premises. Without
          limiting the foregoing, if the presence of any Hazardous Material on
          the Premises results in any contamination of the Premises, Lessee
          shall promptly take all actions at its sole expense as are necessary
          to return the Premises to a condition which does not constitute
          contamination under applicable Governmental Regulations; provided that
          (x) Lessee is responsible for the introduction of such Hazardous
          Material on the Premises (which for the purposes of this Section 45
          means that Lessee cannot prove by a preponderance of evidence that a
          third party is responsible therefor), and (y) Lessor's approval of
          such actions shall first be obtained, which approval shall not be
          unreasonably withheld so long as such actions would not have any
          potential to cause a material adverse long-term or 

                                     -48-
<PAGE>
 
          short-term effect on the Premises. The appropriate remediation plan to
          return the Premises to such level shall conclusively be determined by
          an environmental consultant designated by Lessor in its sole and
          absolute discretion. The parties hereto agree that to the extent there
          exists any inconsistencies between the indemnification set forth
          herein and the indemnification set forth in the Purchase Agreement,
          the indemnification set forth in this Paragraph 45.1 shall govern.

               (d)  Except for small, inconsequential amounts stored in retail
          containers, within ten (10) days after the Commencement Date, Lessee
          shall deliver to Lessor a detailed list of all of the Hazardous
          Materials which Lessee is likely to use, keep store and/or maintain in
          or about the Premises. Lessee shall not cause or permit any other
          Hazardous Material to be brought upon, kept or used in or about the
          Premises by Lessee, its agents, employees, contractors or invitees or
          subtenants unless Lessee provides written notice thereof to Lessor and
          provides reasonable written evidence to Lessor (to Lessor's reasonable
          satisfaction) that such Hazardous Material is necessary or useful to
          Lessee's business and will be used, kept and stored in a manner that
          complies with all Governmental Regulations regulating any such
          Hazardous Material so brought upon or used or kept in or about the
          Premises.

               (e)  Lessor and Lessee acknowledge that Lessor may become legally
          liable for the costs of complying with Governmental Regulations
          relating to Hazardous Material which may not be the legal
          responsibility of Lessee, including but not limited to the following:
          (i) a change in Governmental Regulations which relate to Hazardous
          Material which make that Hazardous Material which is present on the
          Premises, whether known or unknown to Lessee, a-violation of such new
          Governmental Regulation; and (ii) Hazardous Material that Lessee is
          unable to conclusively demonstrate (to a preponderance of the
          evidence) has migrated, flowed, percolated, diffused or in any way
          moved on, to or under the Premises after the Commencement Date from
          adjacent properties. Accordingly, Lessor and Lessee agree that
          compliance with Governmental Regulations (amended or enacted after the
          Commencement Date) relating to Hazardous Material on the Premises for
          which Lessor may be legally liable shall be the sole

                                     -49-
<PAGE>
 
          responsibility of Lessee and, to the event legally permissible, Lessee
          hereby assumes such responsibility and legal liability and forever
          releases and discharges Lessor from the same and agrees that any cost
          incurred by Lessor with respect to the same shall be paid by Lessee on
          demand as additional rental under the Lease and shall be subject to
          the terms of Paragraphs 45.1(b) and 45.1(c) above. Further, Lessee
          shall be solely responsible for Hazardous Material that in any way
          appears in, onto or under the Premises after the Commencement Date;
                           --------
          provided, however, that if Lessee can prove by a preponderance of
          evidence that a third party is responsible for said migration, flow,
          diffusion or appearance, Lessee shall be relieved of all
          responsibility therefor and provided further that Lessee shall not be
                         ------------------------------------------------------
          responsible for Hazardous Material that appears in, onto or under the
          ---------------------------------------------------------------------
          Premises from conduct which occurs after the termination of this
          ----------------------------------------------------------------
          Lease. In the event any such expense relating to Hazardous Material is
          -----
          subsequently recovered or reimbursed through insurance, or recovery
          from responsible third parties, or other action, Lessee shall be
          entitled to reimbursement to the extent it has paid such expense to
          which such recovery or reimbursement relates.

                (f) Lessee shall immediately notify Lessor of any inquiry, test,
          investigation, or enforcement proceeding concerning a Hazardous
          Material by or against Lessee or the Premises. Further, Lessee shall
          make available to Lessor upon demand, copies of any tests,
          investigations and studies undertaken by or at the request of Lessee
          relating to Hazardous Materials (including, if Lessee disputes its
          responsibility for the presence of any Hazardous Material on the
          Premises) Lessee's purchase orders and manifests to ascertain what
          Hazardous Materials have been brought by Lessee to the Premises.
          Lessee acknowledges that Lessor may, at its election and at Lessee's
          expense, negotiate, defend, approve, and appeal any action taken or
          order issued with regard to a Hazardous Material by an applicable
          Governmental Authority.

          45.2  Hazardous Material Definition. As used herein, the term
                -----------------------------
Hazardous Material means any hazardous or toxic substance, material or wastes
which is or becomes regulated by any local governmental authority, the State or
the United States Government. The term "Hazardous Material. includes, without
limitation, any material or substance which is (i) petroleum, (ii) asbestos,
(iii) designated as a "hazardous substance"

                                     -50-
<PAGE>
 
pursuant to Section 311 of the Federal Water Pollution Control Act (33 U.S.C.
(S) 1317), (iv) defined as a "hazardous waste" pursuant to Section 1004 of the
Federal Resource Conservation and Recovery Act, 42 U.S.C. (S) 6901 et seq. (42
U.S.C. (S) 6903), or (v) defined as a "hazardous substance. pursuant to Section
101. of the Comprehensive Environmental Response, Compensation and Liability
Act, 42 U.S.C. (S) 9601 EL seq. (42 U.S.C. (S) 9601).

          45.3  Governmental Regulations Defined. "Governmental Regulations"
                --------------------------------
means any laws, ordinances, rules, requirements, resolutions, policy statements
and regulations (including, without limitation, those relating to land use,
subdivision, zoning, environmental, toxic or hazardous waste, occupational
health and safety, water, earthquake hazard reduction, and building and fire
codes) of any governmental or quasi-governmental body or agency having
jurisdiction over the Premises, Lessor or Lessee, bearing on the construction,
alteration, rehabilitation, maintenance, leasing, use, operation or sale of the
Premises.

          45.4  Underground Storage Tanks. At least every two (2) years during
                -------------------------
the term of this Lease, or any extension thereof, Lessee shall test the
underground storage tanks located on the Premises for integrity and freedom from
leakage. The tests shall be performed by a contractor with expertise in testing
underground storage tanks and the written results of such tests shall be
provided to Lessor. Lessee shall comply with the requirements set forth in 40
C.F.R. parts 280.40, 280.41 and 280.43 in connection with such tanks.

          45.5  Groundwater Monitoring. Within six (6) months after the date of
                ----------------------
this Lease, Lessee shall install on the Premises, at its own expense and in
accordance with plans previously approved by Lessor (such approval not to be
unreasonably withheld), applicable law and good industry practice, permanent
groundwater monitoring wells or early-warning monitoring devices in connection
with all underground tanks on the Premises. Any monitoring wells shall be of a
minimum depth of thirty (30) feet and Lessee shall maintain such wells or
monitoring devices in good repair throughout the term of this Lease. Lessee
shall cause a testing organization previously approved by Lessor (such approval
not to be unreasonably withheld) to take and analyze samples from such wells
semi-annually (quarterly after any test which reveals the presence of Hazardous
Materials) with a sampling range from ten (10) feet to the bottom of each well.
All analyses performed pursuant to this Paragraph 45.5 should at least
specifically check such samples for petroleum hydrocarbons using at least EPA
method 418.1 (or a comparable replacement method).

                                     -51-
<PAGE>
 
          45.6  Survival.  The provisions of this Section 45 shall survive the
                --------                                                      
expiration or sooner termination of this Lease by either Lessor or Lessee.


                                  SECTION 46

                                     LIENS

          Lessee shall not suffer any lien to be recorded against the Premises
as a consequence of a Hazardous Material, including any so-called state, federal
or local "super fund" lien relating to the clean-up of a Hazardous Material in,
on or about the Premises; provided, however, that in the event that a lien is
recorded against the Premises as a result of any act of Lessee, Lessee shall
promptly and diligently perform any and all acts necessary to cause the removal
of such lien.


                                  SECTION 47

                     LESSOR'S RIGHT TO RELEASE EXCESS LAND

          47.1  Release of Excess Land. On and after one year after the
                ----------------------
Commencement Date, at any time and from time to time, Lessee shall, within ten
(10) days after receipt of a written request from Lessor, execute and
acknowledge an instrument or instruments in recordable form reasonably
acceptable to Lessor terminating this Lease with respect to all or any portion
of the real property described on Exhibit "C" attached hereto (the "Excess
                                  -----------
Land"). Those portions of the Excess land released from time to time pursuant to
this Paragraph 47.1 are hereinafter collectively referred to as the "Released
Land". Lessor may exercise the right described in the immediately preceding
sentence as many times as Lessor desires without exhausting such right. Upon the
termination of this Lease with respect to the Released Land at any time, this
Lease shall be deemed null, void and of no force and effect with respect to the
Released Land and such Released Land shall not be considered part of the
Premises. In no event shall the termination of-this Lease with respect to any
Released Land cause or permit the reduction of any Rent otherwise due under this
Lease.

          47.2  Development of Excess Land. Lessee acknowledges and understands
                --------------------------
that Lessor may further develop the Released Land following the termination of
this Lease with respect to such Released Land. In connection with such

                                     -52-
<PAGE>
 
development, Lessor shall have the right to construct, remove and/or relocate
any and all roadways, driveways, utility lines and easements, whether located on
the Premises, on the Excess Land, or the Released Land without the consent of
Lessee; provided, however, that at all times Lessee shall have adequate
pedestrian and vehicular access to, adequate utility service at, and adequate
parking on, the Premises. Lessee agrees to execute and acknowledge, promptly
upon request by Lessor, any instruments requested by Lessor reasonably necessary
to evidence the subordination of this Lease to any new access, utility,
temporary construction or other easements over the Premises reasonably requested
by Lessor in connection with the development of the Released Land. Lessee
further agrees to cooperate with Lessor in connection with all such development
provided that Lessee shall not be required to expend any funds in connection
therewith.


                                  SECTION 48

                                FOREIGN BUYERS

                                     -53-
<PAGE>
 

                          [Page intentionally blank]






                                     -54-
<PAGE>
 
LESSOR AND LESSEE HAVE CAREFULLY READ AND REVIEWED THIS LEASE AND EACH TERM AND
PROVISION CONTAINED HEREIN AND, BY EXECUTION OF THIS LEASE, SHOW THEIR INFORMED
AND VOLUNTARY CONSENT THERETO. THE PARTIES HEREBY AGREE THAT, AT THE TIME THIS
LEASE IS EXECUTED, THE TERMS OF THIS LEASE ARE COMMERCIALLY REASONABLE AND
EFFECTUATE THE INTENT AND PURPOSE OF LESSOR AND LESSEE WITH RESPECT TO THE
PREMISES.


                                        UMT RESTON, INC.,
                                        a Delaware corporation

On March 29, 1990

Executed at Blue Bell, PA               By:   [SIGNATURE ILLEGIBLE]
                                            -----------------------------

                                        Title:         PRESIDENT
                                               --------------------------

Address:                                               "Lessor"

c/o Coldwell Banker Capital
Management Services
533 Fremont Avenue
Los Angeles, California 90071
Attn: Mr. Fred C Wasson

with a copy to:

c/o Coldwell Banker Capital
Management Services
533 Fremont Avenue
Los Angeles, California 90071
Attn: Mr. Ross E. Turner

                                     -55-
<PAGE>
 
                                        UNISYS CORPORATION,
                                        a Delaware corporation

On March 25, 1990


Executed at Blue Bell, PA               By:  /s/ James W. Olson
                                            --------------------------        
                                                 James Olson
                                        Title:   Vice President

Address:                                             "Lessee"

Unisys Corporation
Township Line and
  Union Meeting Roads
P.O. Box 500
Blue Bell, Pennsylvania 19424-0001
Attn:  Real Estate Department

with a copy to:

Unisys Corporation
Law Department
Township Line and
  Union Meeting Roads
P.O. Box 500
Blue Bell, Pennsylvania 19424-0001

                                     -56-