Document:

IMPORTANT NOTE: CERTAIN MATERIAL, INDICATED BY [***], HAS BEEN OMITTED FROM THIS
DOCUMENT PURSUANT TO A REQUEST FOR CONFIDENTIAL TREATMENT.

                                 DEED OF LEASE

                                     BETWEEN

        11720 Sunrisecorp., L.L.C., a Maryland limited liability company

                                  as Landlord,

                                       AND

                      PATHNET, INC. a Delaware corporation

                                   as Tenant.

                             Dated: November 1, 1999

                              For Premises Located

                         At: 11720 Sunrise Valley Drive
                                Reston, Virginia
                        Terrace, First and Second Floors

<PAGE>

                                  DEED OF LEASE

         THIS  DEED  OF  LEASE  (this  "Lease")  is  made  as of the  1st day of
November, 1999 (the "Date of Lease"), by 11720 Sunrisecorp.,  L.L.C., a Maryland
limited liability company ("Landlord"), and Pathnet, Inc. a Delaware corporation
("Tenant").

         Landlord and Tenant, intending legally to be bound, hereby covenant and
agree as set forth below.

                                    ARTICLE I
                             BASIC LEASE PROVISIONS

         The  following  terms,  when used  herein,  shall have the meanings set
forth below.

         1.1 PREMISES.  For purposes of this Lease,  the rentable square footage
of the Premises  shall be deemed to be 39,103  square feet, on the Ground Floor,
First Floor and Second Floor of the Building  (hereafter defined) as outlined on
Exhibit A-1 attached hereto and made a part hereof.  For purposes of this Lease,
the Premises shall consist of two (2) portions, the first ("Space A") consisting
of approximately  10,157 rentable square feet located on the ground floor of the
Building as shown on EXHIBIT A-1,  attached  hereto and  incorporated  herein by
reference,  and the  second  ("Space  B")  consisting  of  approximately  28,946
rentable  square feet on the first and second floors of the Building as shown on
EXHIBIT A-1. During the period between the Space A Commencement Date (as defined
below) and the Space B Commencement Date (as defined below),  the Premises shall
be deemed to consist solely of Space A. After the Space B Commencement Date, the
Premises  shall  be  deemed  to  consist  of  Space A and  Space  B.  Applicable
provisions of this Lease shall be construed accordingly,  to give effect to such
phased delivery of the Premises. The Premises shall include the exclusive use of
the loading dock that is adjacent to (and connected to) Space A.

         1.2 BUILDING.  The building  containing  approximately  68,771 rentable
square feet shown on EXHIBIT A-2 attached hereto and made a part hereof, and all
alterations,  additions,  improvements,  restorations  or  replacements  now  or
hereafter made thereto, with an address of:

                  11720 Sunrise Valley Drive
                  Reston, Virginia

The Premises and Building  have been  measured  prior to the Date of Lease using
the BOMA  standard  method of  measurement,  and the square  footages  set forth
herein are hereby stipulated by the parties.

         1.3 TERM.  Commencing on the Space A Commencement Date (with respect to
Space A) and on the Space B  Commencement  Date  (with  respect  to Space B) and
ending on the Expiration Date.

         1.4 COMMENCEMENT  DATE. With respect to Space A, November 22, 1999 (the
"Space A Commencement Date"). Except as specifically provided to the contrary in
EXHIBIT B, with respect to Space B, March 15, 2000.

         1.5 EXPIRATION DATE. The last day of the [***] Lease Year, as such term
is defined below.

         l.6     BASE RENT.
         (a) A total of [***] for the first Lease Year payable in equal  monthly
installments of [***]. The Base Rent shall be increased  annually  commencing on
the  first  day of the  second  Lease  Year in  accordance  with  the  following
schedule:

<PAGE>

         LEASE YEAR        TOTAL ANNUAL BASE RENT    MONTHLY INSTALLMENT
-------------------     ------------------------------------------------
         [***]             [***]                     [***]

Except as hereafter  provided,  the above rent schedule shall take effect on the
first day of the first Lease Year, which shall be April 1, 2000 unless the Space
B Commencement Date is deferred past April 1, 2000 pursuant to the express terms
of EXHIBIT B, in which  event the first day of the First Lease Year shall be the
Space B Commencement  Date unless the Space B  Commencement  Date is a day other
than the first day of a calendar month, in which case the first day of the first
Lease Year shall be the first day of the calendar  month  occurring  immediately
after the Space B Commencement  Date. In addition to Base Rent payable under the
above rent schedule, Tenant shall make a payment to Landlord on April 1, 2000 in
respect  of its use and  occupancy  of Space A in the  amount of [***]  (for the
period prior to and including March 31, 2000). If the Space B Commencement  Date
is for any reason  deferred  past April 1, 2000 pursuant to the express terms of
EXHIBIT B, Tenant will  beginning  on April 1, 2000,  pay Monthly  Base Rent for
Space A at a rate of [***] per  month  for  Space A,  until the first day of the
first  Lease  Year,  at which time the above rent  schedule  shall  apply to the
Premises as a whole. In addition, on the Space B Commencement Date, Tenant shall
pay Base Rent for Space B for the period between the Space B  Commencement  Date
and the first day of the first  Lease  Year at the same per square  foot  rental
rate as in effect for the first Lease Year, calculated for Space B on a PER DIEM
basis.  By way of example,  if the Space B Commencement  Date is March 15, 2000,
Tenant will,  in addition to the [***] payment for Space A which is due on April
1, 2000,  and the Base Rent payment of [***] which is due on April 1, 2000 under
the above rent  schedule,  make a payment of Base Rent for Space B in the amount
of  [***],  for the  17-day  period  from  March  15,  2000 to March  31,  2000,
inclusive, which payment will be due and payable on March 15, 2000.

         1.7 SECURITY DEPOSIT.  Initially,  the Security Deposit shall be [***].
Landlord  agrees that,  provided  Tenant has not  committed a Default under this
Lease (after giving effect to all  applicable  notice and cure periods) which is
then  continuing,  and has not committed more than [***] Defaults  (after giving
effect to all applicable  notice and cure periods) within the [***] month period
prior to the date of such reduction (even if the same are not then  continuing),
the  Security  Deposit  shall be subject to  reduction  on an annual  basis,  in
accordance with the following schedule:

                       REDUCTION DATE            AMOUNT OF SECURITY DEPOSIT
---------------------------------------------------------------------------
                         [***]                               [***]

After the first day of the [***] Lease Year, there will be no further  reduction
of the Security Deposit,  which will remain at [***] for the balance of the Term
(as the same may be extended).  In addition,  if Tenant  commits more than [***]
Defaults under this Lease (after giving effect to all applicable notice and cure
periods) in any [***] period, there shall thereafter be no further reductions of
the Security Deposit.

         1.8 BASE YEAR. The Base Year for calculation of Operating  Expenses and
Real Estate Taxes shall be Calendar Year 2000 (E.G., January 1, 2000 to December
31, 2000).

         1.9 TENANT'S  PROPORTIONATE SHARE OF OPERATING EXPENSES.  56.86% of the
Operating Expenses.

         1.10 TENANT'S  PROPORTIONATE  SHARE OF REAL ESTATE TAXES 56.86% of Real
Estate Taxes .

         1.11  PARKING  SPACE  ALLOCATION.  3.3 spaces per 1,000  square feet of
rentable  area,  which  shall be in  unreserved,  non-exclusive  parking  spaces
available in the Parking Facilities.

         1.12 PERMITTED USE. Tenant may use the Premises for general office uses
and any  other  use  permitted  as a matter  of right  under  applicable  zoning
regulations  which is not a  "Prohibited  Use", as hereafter  defined.  Landlord
agrees that the term  "Permitted Use" shall include any other use permitted as a
matter of right under  applicable  zoning  regulations  which is appropriate and
incidental  to the  operation  of a  telecommunications  provider,  including  a
network operating center,  testing facilities for electronic equipment,  network
interconnection space, and training facilities.  The foregoing  notwithstanding,
Tenant agrees that the following uses shall be prohibited,  even if permitted as

<PAGE>

a matter of right under  applicable  zoning  regulations  or related to Tenant's
business as a  telecommunications  provider  (each being referred to herein as a
"Prohibited  Use"):  (i) Any retail use; (ii) Any industrial  use; (iii) Any use
which would  involve the  storage,  disposal,  use or  processing  of  Hazardous
Materials  (other  than  Permitted  Materials  which  are  specifically  allowed
herein); (iv) any use involving unreasonable safety risks or concerns (including
any  significant  risk of bodily injury or property  damage);  (v) any use which
would increase the rates charged for property and casualty insurance  applicable
to the Building;  and (vi) any use which is not consistent with the operation of
a first class  office  property,  as  determined  by Landlord in its  reasonable
discretion.  Any use or  activity  which  is not a  Permitted  Use is  expressly
prohibited by this Lease.

         1.13     TENANT'S TRADE NAME. Pathnet

         1.14     BROKER(S).  Landlord's: Julien J. Studley, Inc.
                              Tenant's: Trammell Crow Real Estate Services, Inc.

         1.15     LANDLORD'S ADDRESS.

         Prior to November 12, 1999:   c/o Penzance Properties, L.L.C.
                                       1719 Hoban Road, N.W.
                                       Washington, D.C. 20007

         November 12, 1999 and after:  c/o Penzance Properties, L.L.C.
                                       5627 Potomac Avenue, N.W.
                                       Washington, D.C. 20016

                  With a copy to:      McShea Management, Inc.
                                       One Bank Street, Suite 300
                                       Gaithersburg, Maryland 20878
                                       Attention: Property Manager
                                                  - 11720 Sunrise Valley Drive

                  And a copy to:       Mark S. Tenenbaum, Esq.
                                       Tenenbaum & Saas, P.C.

<PAGE>

                                       4330 East-West Highway
                                       Suite 1150
                                       Bethesda, MD 20814

         1.16    TENANT'S ADDRESS.

                  Before Occupancy: Pathnet, Inc.
                                            1015 31st Street, N.W.
                                            Washington, D.C. 20007
                                            Attn: General Counsel

                  After Occupancy:  Pathnet, Inc.
                                            11720 Sunrise Valley Drive
                                            Reston, VA [ZIP]
                                            Attention: General Counsel

         1.17     GUARANTOR AND GUARANTOR'S ADDRESS.  Not Applicable.

                                    ARTICLE 2
                                   DEFINITIONS

         The  following  terms,  when used  herein,  shall have the meanings set
forth below.

         2.1 ADDITIONAL RENT. As defined in Section 5.3.

         2.2 AGENTS.  Officers,   partners,   directors,   employees,   agents,
licensees, customers, contractors and invitees.

         2.3 ALTERATIONS.  Alterations, additions or improvements of any kind or
nature to the Premises or the Building,  whether  structural or  non-structural,
interior,  exterior  or  otherwise  except  that,  with  respect  to  any of the
foregoing  items  proposed  to be made for or by  Tenant,  Alterations  shall be
deemed to exclude (i)  decorations of a cosmetic  nature which do not impact the
structure or operation of the base building systems of the Building and will not
be readily  apparent  and  visible  from the  exterior  of the  Building  (e.g.,
carpeting,  wall coverings,  painting),  (ii) Tenant's equipment,  furniture and
demountable wall panels (but Alterations  shall not exclude  Landlord's right of
review over the integration of such equipment, furniture and/or demountable wall
panels to any Building  utility  system or structural  elements),  and (iii) any
Tenant Improvements constructed by Landlord pursuant to EXHIBIT B.

         2.4 CALENDAR  YEAR. A period of twelve (12) months  commencing  on each
January 1 during the Term,  except  that the first  Calendar  Year shall be that
period from and including the Commencement Date through December 31 of that same
year,  and the last  Calendar  Year shall be that period from and  including the
last January 1 of the Term through the earlier of the Expiration Date or date of
Lease termination. Notwithstanding the foregoing, the Calendar Year for the Base
Year shall be not more than, and not less than, one (1) year.

         2.5 COMMON AREA. All areas, improvements, facilities and equipment from
time to time  designated  by  Landlord  for the common use or benefit of Tenant,
other tenants of the Building and their Agents,  including,  without limitation,
roadways,  entrances  and exits,  landscaped  areas,  open  areas,  park  areas,
exterior  lighting,   service  drives,   loading  areas,   pedestrian  walkways,
sidewalks,   atriums,   courtyards,   concourses,   stairs,  ramps,   washrooms,
maintenance  and utility rooms and closets,  exterior  utility lines,  hallways,
lobbies,  elevators and their housing rooms,  common window areas, common walls,
common ceilings, common trash areas and Parking Facilities.

         2.6  EVENT OF DEFAULT. As defined in Article 22.
<PAGE>

         2.7  HEREIN,  HEREAFTER,   HEREUNDER  AND  HEREOF.  Under  this  Lease,
including, without limitation, all Exhibits and any Riders.

         2.8 INCLUDING  AND/OR INCLUDES.  Including,  but not limited to, and/or
includes, without limitation. The term "including" and like terms used herein to
identify  particular  items as being  related to (or examples of) a  descriptive
term,  phrase or  provision  shall,  unless  where  expressly  indicated  to the
contrary,  be viewed as  non-exclusive,  and the  identification  of  particular
items, or use of such examples,  shall not limit any other item or example which
fits within the particular descriptive term, phrase or provision.

         2.9 INTEREST  RATE.  Per annum interest rate listed as the base rate on
corporate  loans at large U.S. money center  commercial  banks as published from
time to time under "Money  Rates" in the WALL STREET  JOURNAL plus three percent
(3 %). but in no event  greater than the maximum  rate  permitted by law. In the
event the WALL  STREET  JOURNAL  ceases to publish  such rates,  Landlord  shall
choose at Landlord's sole discretion a similar  publication which publishes such
rates.

         2.10 LAND. The piece or parcel of land described in EXHIBIT A-2 and all
rights,  easements and appurtenances thereunto belonging or pertaining,  or such
portion thereof as shall be allocated by Landlord to the Building.

         2.11 LEASE YEAR.  Each  consecutive  twelve (12) month period  elapsing
after (i) the Space B Commencement Date, if the Space B Commencement Date occurs
on the first day of a month,  or (ii) the first day of the month  following  the
Space B Commencement  Date, if the Space B  Commencement  Date does not occur on
the first day of a month.

         2.12 MORTGAGE. Any mortgage,  deed of trust, security interest or title
retention interest affecting the Building or the Land.

         2.13  MORTGAGEE.  The  holder of any note or  obligation  secured  by a
mortgage, deed of trust, security interest or title retention interest affecting
the Building or the Land,  including,  without limitation,  lessors under ground
leases, sale-leasebacks and lease-leasebacks.

         2.14 OPERATING EXPENSES. As defined in Section 7.2.

         2.15  PARKING  FACILITIES.  All  parking  areas now or  hereafter  made
available  by  Landlord  for  use by  tenants,  including,  without  limitation,
open-air parking,  parking decks and parking areas under or within the Building,
whether reserved, exclusive, non-exclusive or otherwise.

         2.16 REAL ESTATE TAXES As defined in Article 8.

         2.17 RENT. Base Rent and Additional Rent.

         2.18  RULES AND  REGULATIONS.  The rules and  regulations  set forth in
EXHIBIT C attached hereto and made a part hereof,  as the same may be amended or
supplemented from time to time.

         2.19 SUBSTANTIAL COMPLETION.  As defined in the Work Agreement attached
hereto and made a part hereof as EXHIBIT B.

         2.20  SUBSTANTIAL  PART.  More than fifty percent (50%) of the rentable
square feet of the Premises or the Building, as the case may be.

         2.21 WORK AGREEMENT. As set forth in EXHIBIT B attached hereto and made
a part of
<PAGE>

                                    ARTICLE 3
                                  THE PREMISES

         3.1 LEASE OF PREMISES.  In  consideration  of the agreements  contained
herein,  Landlord hereby leases the Premises to Tenant, and Tenant hereby leases
the  Premises  from  Landlord,  for the Term and upon the terms  and  conditions
hereinafter provided. As an appurtenance to the Premises,  Tenant shall have the
non-exclusive  right,  together  with other  tenants of the  Building  and their
Agents,  to use the Common Area.  Landlord  shall retain  absolute  dominion and
control  over the Common  Area and shall  operate  and the  Common  Area in such
manner as Landlord, in its sole discretion, shall determine;  provided, however,
such exclusive right shall not operate to prohibit or materially  interfere with
Tenant's  Permitted Use and quiet enjoyment of the Premises.  Landlord expressly
reserves the right permanently to change, modify or eliminate, or temporarily to
close,  any  portion of the Common  Area,  provided  Landlord  will not make any
permanent (or long term) modifications which materially  interfere with Tenant's
access to the Premises,  with Tenant's business  operations in the Premises,  or
with Tenant's other rights under this Lease. The Premises are leased subject to,
and Tenant agrees not to violate,  all present and future covenants,  conditions
and restrictions of record which affect the Building and Land.

         3.2  LANDLORD'S  RESERVATIONS.  In  addition  to the  other  rights  of
Landlord under this Lease,  Landlord reserves the right (i) to change the street
address and/or name of the Building,  (ii) to install,  erect, use, maintain and
repair mains, pipes,  conduits and other such facilities to serve the Building's
tenants in and  through  the  Premises,  (iii) to grant to anyone the  exclusive
right to conduct any particular business or undertaking in the Building, (iv) to
establish a condominium regime for the Building, the Land and/or the Common Area
and to include the Premises  therein,  (v) subject to Section  29.2,  below,  to
control the use of the roof and  exterior  ways of the Building for any purpose,
and (vi) to modify the size and configuration of the Common Area,  including the
construction  of temporary  or permanent  structures  or  improvements  therein,
provided Landlord will not make any permanent (or long term) modifications which
materially  interfere  with  Tenant's  access  to the  Premises,  with  Tenant's
business  operations in the Premises,  or with Tenant's  other rights under this
Lease. Subject to the foregoing limitations, Landlord may exercise any or all of
the foregoing rights without being deemed to be guilty of an eviction, actual or
constructive,  or a  disturbance  or  interruption  of the business of Tenant or
Tenant's use or occupancy of the Premises.

                                    ARTICLE 4
                                      TERM

         The Term shall commence on the Space A Commencement  Date (with respect
to Space A) and on the Space B  Commencement  Date (with respect to Space B) and
shall  expire at midnight on the  Expiration  Date.  If  requested  by Landlord,
Tenant  shall  within  fifteen  (15)  days of such  request  sign a  declaration
acknowledging the Commencement Date and the Expiration Date in the form attached
hereto and made a part hereof as EXHIBIT D.

                                    ARTICLE 5
                                      RENT

         5.1  BASE RENT. Tenant shall pay to Landlord the Base Rent as specified
in Section 1.6.

         5.2  PAYMENT  OF BASE  RENT.  Base Rent for each  Lease  Year  shall be
payable in monthly installments,  in advance, without demand, notice, deduction,
offset or  counterclaim,  on or before the first day of each and every  calendar
month during the Term, subject however,  to the provisions of Section 1.6, above
in relation to Base Rent payable for the period between the Space A Commencement
Date  and  the  first  day of  the  first  calendar  month  after  the  Space  B
Commencement  Date.  Tenant shall pay the Base Rent and all Additional  Rent, by
good check,  made payable to Landlord or in lawful currency of the United States
of America, to Landlord at c/o McShea Management,  Inc., One Bank Street,  Suite
300,  Gaithersburg,  Maryland  20878,  or to such other address or in such other
manner as Landlord from time to time specifies by written notice to Tenant.  Any
payment  made by Tenant to  Landlord  on account of Base Rent may be credited by
Landlord to the payment of any late charges then due and payable and to any Base
Rent or  Additional  Rent  then  past due  before  being  credited  to Base Rent
currently due.

         5.3 ADDITIONAL RENT. All sums payable by Tenant under this Lease, other
than Base Rent,  shall be deemed  'Additional  Rent,' and, unless  otherwise set
forth  herein,  shall be payable in the same  manner as set forth above for Base
Rent.
<PAGE>

         5.4  ACCEPTANCE  OF RENT.  If Landlord  shall direct Tenant to pay Base
Rent or Additional Rent to a 'lockbox' or other depository whereby checks issued
in payment  of Base Rent or  Additional  Rent (or both,  as the case may be) are
initially  cashed or deposited by a person or entity other than Landlord (albeit
on  Landlord's  authority),  Tenant  agrees  to make such  payments  in a timely
fashion in accordance with such direction from Landlord.

                                    ARTICLE 6
                                SECURITY DEPOSIT

         6.1 GENERAL.  Simultaneously  with the execution of this Lease,  Tenant
shall  deposit  in the form of an  irrevocable  letter  of credit  the  Security
Deposit  with  Landlord,  which shall be held by  Landlord  as security  for the
performance  of Tenant's  obligations  and  covenants  under this  Lease.  It is
expressly  understood  and agreed  that such  deposit  is not an advance  rental
deposit or a measure of Landlord's damages in case of an Event of Default.

         6.2 SECURITY  AFTER HAVING BEEN  CONVERTED INTO CASH. If Landlord shall
draw  upon any  letter  of credit  provided  by  Tenant as and for its  Security
Deposit in accordance with this Lease,  thereby converting such letter of credit
into cash,  the terms of this  Section 6.2 shall  apply.  If an Event of Default
shall  occur or if Tenant  fails to  surrender  the  Premises  in the  condition
required by this Lease,  Landlord shall have the right (but not the obligation),
and without  prejudice to any other  remedy  which  Landlord may have on account
thereof,  to apply  all or any  portion  of the  Security  Deposit  to cure such
default or to remedy the condition of the  Premises.  If Landlord so applies the
Security  Deposit or any portion  thereof before the Expiration  Date or earlier
termination of this Lease, Tenant shall deposit with Landlord,  upon demand, the
amount  necessary to restore the Security  Deposit to its  original  amount.  If
Landlord  shall sell or transfer its interest in the  Building,  Landlord  shall
have the right to transfer the Security Deposit to such purchaser or transferee,
and upon such  transfer,  Tenant  shall look solely to the new  landlord for the
return of the Security  Deposit,  and Landlord  thereupon shall be released from
all liability to Tenant for the return of the Security Deposit.  Whether held in
the form of cash or in the form of a letter of credit,  any remaining balance of
the Security  Deposit  shall be returned to Tenant within thirty (30) days after
the  Expiration  Date or earlier  termination  of this  Lease,  less any amounts
retained or permitted to be held thereafter by Landlord pursuant to the terms of
this Lease pending the  satisfaction of Tenant's  obligations  under this Lease.
Any portion of the  Security  Deposit that is held in cash or other liquid funds
(as  opposed  to in the form of a letter  of  credit)  shall  be  maintained  by
Landlord in an account with a third-party bank or financial institution at which
Landlord   normally  conducts  its  banking  (or  as  designated  by  Landlord's
mortgagee),  shall  be  invested  only  in  direct  obligations  of the  federal
government, in an escrow account segregated from Landlord's other funds, and any
interest earned thereon shall be deemed to have been added to and to form a part
of the  Security  Deposit (and shall be reflected as income to Tenant for income
tax reporting  purposes),  PROVIDED  Tenant shall have the right to request (and
receive) a disbursement  of such interest on a quarterly basis (as of the end of
each  calendar  quarter)  so  long  as it is not in  Default  hereunder  (beyond
expiration of applicable notice and cure periods).  Tenant's EIN for purposes of
federal income tax reporting is _____________.

         6.3  SECURITY IN THE FORM OF AN  IRREVOCABLE  LETTER OF CREDIT.  If the
Security  Deposit  is posted in the form of a letter of credit,  such  letter of
credit  (i) shall be  unconditional  and  irrevocable,  (ii)  shall  name as the
beneficiary Landlord and any party reasonably  designated by Landlord (including
Landlord's mortgagee),  either of whom may act without joinder of the other, and
shall be fully assignable by the beneficiary, (iii) shall permit multiple draws,
(iv) shall be issued by a commercial bank reasonably acceptable to Landlord with
branches in the  Washington,  D.C.  metropolitan  area,  (v) shall be payable at
sight upon  presentment  to a local  branch of the issuer in in the  Washington,
D.C.  metropolitan  area, of a simple sight draft signed by any one  beneficiary
accompanied  by a  certificate  signed  by such  beneficiary  stating  that  the
Landlord is permitted to draw upon such Letter of Credit under the express terms
of this Lease, either because the Tenant is in Default hereunder,  or has failed
to satisfy the  requirements  of clause (vii) of this Section  6.3,  below,  and
setting  forth the amount that is permitted to be drawn in such  instance,  (vi)
shall  have a term of not less than one (1)  year;  and  (vii)  shall,  at least
thirty  (30) days prior to the  then-current  expiration  date of such Letter of
Credit,  either (a) be renewed (or automatically and  unconditionally  extended)
for another period of not less than one year, continuing throughout the Term, so
that the same is  maintained  in full force and  effect  through  the  thirtieth
(30th) day after expiration of the Lease Term), or (b) replaced with cash in the
amount of the  Security  Deposit.  In the event of Default  by Tenant  hereunder
(E.G., after expiration of applicable notice and cure periods),  or a failure to
renew or extent the Letter of credit or supply a cash Security  Deposit in place
thereof pursuant to clause (vii), above, Landlord shall have the right to redeem
or draw upon all or any  portion of such  letter of credit in order to cure such
default (including satisfaction of any sums due and owing from Tenant under this
Lease) by  submitting a sight draft to the  financial  institution  issuing such
letter of credit  indicating  the amount  sought to be drawn,  and stating  that
Landlord is permitted to draw upon the letter of credit pursuant to the terms of
this Lease in the amount  indicated.  Notwithstanding  anything in this Lease to
the contrary,  any grace period or cure periods  which are otherwise  applicable
under Section 24, hereof, shall not apply to Tenant's failure to comply with the
requirements of clause (vii), above, or if the Letter of Credit is set to expire
under circumstances where Landlord is permitted to continue to hold the Security
Deposit under Section 6.5, below past such  expiration  date, and in such event,
Landlord  shall  have the  immediate  right to draw upon the Letter of Credit in
full and hold the proceeds thereof as a cash Security Deposit in accordance with
the terms  hereof.  In the event any  portion of such  letter of credit has been
redeemed or drawn upon by  Landlord,  Tenant  shall be required to restore  such
letter of credit to its full amount as provided  herein.  Any failure or refusal
of the issuer to honor the Letter of Credit  shall be at Tenant's  sole risk and
shall not  relieve  Tenant of its  obligations  hereunder  with  respect  to the
Security  Deposit.  Within  thirty (30) days after the  expiration  of the Term,

<PAGE>

Landlord  shall  return such letter of credit to Tenant,  subject to  Landlord's
right to draw  sums  thereunder  to secure  the  completion  of any  unperformed
obligations  of Tenant  under  this  Lease,  as more fully set forth  below.  If
Landlord  shall sell or transfer its interest in the  Building,  Landlord  shall
have the right to  obligate  Tenant to amend said letter of credit so that it is
payable to such purchaser or transferee, in which event Tenant shall look solely
to such  purchaser  or  transferee  for the return of the letter of credit,  and
Landlord thereupon shall be released from all liability to Tenant for the return
of the Security Deposit.

         6.4 POST MOVE-OUT INSPECTION. Landlord and Tenant shall conduct a "Post
Move-Out  Inspection" of the Premises at a time reasonable scheduled by Landlord
and  Tenant  within  fifteen  (15) days  after the  Expiration  Date or  earlier
termination of this Lease, but in any event prior to reentry by Landlord for the
purpose of preparing the Premises for relet.  Failure on Tenant's part to attend
the Post  Move-Out  Inspection,  after notice from Landlord of the exact date(s)
thereof,  within said fifteen (15) day period shall be deemed an  acceptance  by
Tenant of Landlord's assessment of the condition of the Premises.

         6.5  RETENTION  OF  SECURITY   DEPOSIT  AFTER   EXPIRATION  OR  EARLIER
TERMINATION OF LEASE.  Landlord  generally agrees to return the Security Deposit
to Tenant within thirty (30) days after the Expiration  Date (or date of earlier
termination of this Lease). The foregoing notwithstanding:  (i) if this Lease is
terminated  due to a Default by Tenant,  Landlord  may  continue to hold (and as
necessary,  apply)  the  Security  Deposit  until  such time as all of  Tenant's
obligations  under this Lease have been  satisfied,  at which time Landlord will
return any  remaining  amount of the Security  Deposit  within  thirty (30) days
after  the  final  satisfaction  of such  obligations,  and (ii) if Tenant is in
breach of any of its  covenants  and/or  obligations  under  this Lease as of or
after the  Expiration  Date (or earlier  termination  of this Lease),  including
without limitation any of Tenant's removal and/or restoration  obligations,  and
any of Tenant  covenants under Article 24, Article 26, Section 29.2 and/or 29.3,
hereof,  and  Landlord  either  notified  Tenant  of such  breach  prior  to the
Expiration Date (or date of such earlier termination) or notifies Tenant thereof
within the thirty  (30) day period  after the  Expiration  Date (or date of such
earlier termination), then in such event, Landlord may continue to hold (and, as
applicable,  apply) the  Security  Deposit (or such portion  thereof,  up to the
whole) as Landlord estimates may be necessary to compensate Landlord for damages
associated with such breach or to perform such  obligations on Tenant's  behalf,
such amount to be held until such time as all of Tenant's obligations under this
Lease have been  satisfied,  at which time  Landlord  will return any  remaining
amount  of the  Security  Deposit  within  thirty  (30)  days  after  the  final
satisfaction of such  obligations.  To the extent the Security  Deposit is being
held in the form of a letter  of  credit at such  time,  Landlord  will have the
right to draw upon the letter of credit in such amount  prior to the  expiration
date  thereof,  and to hold  the  amount  so drawn  as a cash  Security  Deposit
pursuant hereto.

<PAGE>

                                    ARTICLE 7
                               OPERATING EXPENSES

         7.1 TENANT'S  PROPORTIONATE SHARES OF OPERATING EXPENSES.  Tenant shall
pay to Landlord throughout the Term, as Additional Rent, Tenant's  Proportionate
Share of the amount by which the  Operating  Expenses  during each Calendar Year
exceed  the  Operating  Expenses  during  the Base  Year.  In the event that the
Commencement  Date or the  Expiration  Date are  other  than the  first day of a
Calendar Year then Tenant's  Proportionate Shares of Operating Expenses shall be
adjusted to reflect the actual period of occupancy during the Calendar Year.

         7.2 OPERATING  EXPENSES  DEFINED.  As used herein,  the term "Operating
Expenses"  shall mean all expenses and costs which Landlord incurs because of or
in connection with the ownership,  maintenance,  management and operation of the
Building and Land. In no event will any gross-up and  allocation  provisions set
forth herein be construed or  implemented  in order to allow Landlord to recover
from the tenants of the Building more than the amounts  expended by Landlord for
the items in question,  and such  provisions  shall be interpreted  equitably to
ensure that the costs and services  which make up the Operating  Expenses of the
Building  are  properly  allocated  among  the  parties  benefiting   therefrom.
Operating Expenses shall include,  without limitation,  all costs,  expenses and
disbursements incurred or made in connection with the following:

                  (i) Wages and  salaries of all  employees,  including  without
limitation an on-site  management agent and staff,  whether employed by Landlord
or the  Building's  management  company  (but,  if  employed  by the  management
company, only to the extent such expense is passed through to Landlord under the
property  management  agreement for the Building),  engaged in the operation and
maintenance  or security of the Building and all costs  related to or associated
with such employees or the carrying out of their duties,  including uniforms and
their cleaning,  taxes,  auto allowances and insurance and benefits  (including,
without  limitation,  customary  contributions  to pension and/or profit sharing
plans and  vacation or other paid leave as customary  for such  employees in the
Northern Virginia  employment market),  PROVIDED that no wages,  salaries and or
related  benefits  and costs will be included  for any  employee at or above the
rank of Vice  President,  and any  such  items  payable  to or with  respect  to
employees (whether on-site or off-site) who do not devote their full time to the
Building  will be included  only to the extent  such  employees  are  reasonably
allocable to the  Building,  as  determined  by the  Landlord in its  reasonable
discretion.

                  (ii) All  supplies and  materials,  including  janitorial  and
lighting  supplies,  used  directly  in the  operation  and  maintenance  of the
Building;

                  (iii)   All   utilities,    including,   without   limitation,
electricity,  telephone,  water,  sewer,  power, gas, heating,  lighting and air
conditioning  for the Building,  except to the extent such utilities are charged
or  chargeable  directly  to, or paid or payable  directly by or on behalf of, a
tenant of the Building or other third party;

                  (iv)  All  insurance   purchased  by  Landlord  or  Landlord's
management  company relating to the Building and any equipment or other property
contained therein or located thereon including,  without  limitation,  casualty,
liability, rental loss, sprinkler and water damage insurance;

                  (v) All  maintenance to the Building  (excluding  repairs paid
for by the proceeds of insurance or by Tenant or other third parties)  including
interior,  exterior,  structural or  non-structural,  and  regardless of whether
foreseen or unforeseen, except as provided in Section 12.1, below, but expressly
excluding capital expenditures or expenses in the nature of capital improvements
except as provided in Section 7.2(xi), below;

                  (vi)  All  maintenance  of the  Building,  including,  without
limitation, painting, ice and snow removal, landscaping,  groundskeeping and the
patching,  painting and  resurfacing  of roads,  driveways and parking lots, but
expressly  excluding  capital  expenditures or expenses in the nature of capital
improvements except as provided in Section 7.2(xi), below;

                  (vii) A  management  fee payable to Landlord or the company or
companies  managing the Building,  if any, not to exceed then prevailing  market
rates  for  managing  comparable  office  buildings  in  the  Reston,   Virginia
submarket;

<PAGE>

                  (viii)  Accounting and legal fees incurred in connection  with
the operation and maintenance of the Building or related thereto;

                  (ix) All maintenance, operation and service agreements for the
Building,  and any equipment  related thereto,  including,  without  limitation,
service and/or maintenance  agreements for the sprinkler system in the Building,
if any (excluding those paid for by Tenant or any other third parties);

                  (x) Any  additional  services  not provided to the Building at
the Commencement Date but thereafter provided by Landlord as Landlord shall deem
necessary or desirable in  connection  with the  management  or operation of the
Building, the Land and the Common Area;

                  (xi) any capital expenditures (1) incurred to reduce Operating
Expenses or utility  costs,  (2) incurred to comply with any  governmental  law,
order,  regulation  or other  requirement  (e.g.,  a  code-mandated  life safety
system)  which are  enacted  or which  become  effective  after the date of this
Lease, or (3) incurred to replace existing equipment and machinery  necessary to
the  day  to  day  operation  of  the  Building,  or  which  constitute  capital
replacements  of common  facilities  serving the Building or Common Areas (i.e.,
replacements  of common area or common  usage  Building  components  and systems
which are necessary,  and are made in lieu of capital repairs otherwise required
to be made thereto) provided that any capital replacement permitted under clause
(3), above (A) shall be undertaken  only where  continued  repair of the item in
question is  inadequate  to remedy the problem as  determined by Landlord in its
reasonable judgment,  (B) shall be amortized on a monthly payment basis over the
useful life of the replacement  item (as determined in accordance with generally
accepted accounting  principles) together with interest at the Interest Rate (or
such higher  interest  rate as may have been paid by Landlord on funds  borrowed
for the purposes of incurring  such  capital  expenditures),  and only the total
monthly  payments  of  principal  and  interest  coming  due in each  applicable
calendar year during the Term, as determined under such  amortization  schedule,
shall be  recoverable  by Landlord under this Section in any such calendar year,
and (C) shall not be included as an Operating Expenses to the extent the same is
made prior to the end of [***] Lease Year.

                  (xiii)  Other  expenses  and costs  reasonably  necessary  for
operating and maintaining the Building, Land and/or Common Area.

         7.3 EXCLUSIONS FROM OPERATING  EXPENSES.  Operating  Expenses shall not
include the following:

                  (i) Legal fees,  space  planners'  fees,  real estate brokers'
leasing  commissions  and advertising  expenses  incurred in connection with the
leasing of space in the Building;

                  (ii)Costs  and  expenses of  alterations  or  improvements  of
the Premises or the leasehold premises of other individual  tenants,  whether by
contribution, rent abatement or otherwise;

                  (iii) Costs of correcting  defects in, or  inadequacy  of, the
design or construction of the Building or the materials used in the construction
of the Building or the equipment or appurtenances  thereto to the extent covered
by warranties and recovered by Landlord;

                  (iv)   Depreciation,   interest  and  principal   payments  on
mortgages  and other debt  costs,  if any,  other than  amortization  of and the
interest factor  attributable to permitted  capital  improvements,  and payments
under any ground leases; and

                  (v) Costs and expenses  associated  with the  operation of the
business  of the person or entity  which  constitutes  Landlord  as the same are
distinguished from the costs of operation of the Building,  including accounting
and  legal  matters,  costs of  defending  any  lawsuits  (other  than  lawsuits
involving  the  maintenance,  repair  and  operation  of Common  Areas,  such as
lawsuits involving vendors or suppliers  providing services or materials used in
connection therewith),  costs of selling or financing any of Landlord's interest
in the  Building  and costs  incurred in  connection  with  disputes  with other
tenants;
                  (vi) Costs and expenses in connection  with services or  other
benefits of a type that are not available to Tenant
without specific charge therefor;

<PAGE>

                  (vii) Penalties,  fines and associated legal expenses incurred
due to  violation  by Landlord or any tenant in the Building of any terms of any
applicable federal, state or local government laws, rules, codes or regulations;

                  (viii) Costs and expenses  otherwise  includable  in Operating
Expenses to the extent that  Landlord is reimbursed  from other  sources  (other
than through  payments of Operating  Expenses by Tenant or other  tenants at the
Building);

                  (ix) Franchise or income taxes imposed on Landlord;

                  (x)  Salaries  paid to  off-site  personnel  of  Landlord  (or
Landlord's property management firm) at or above the level of Vice President, or
otherwise  not  permitted to be included as an Operating  Expense  under Section
7.2(i), above;

                  (xi)  Landlord's  advertising  and  promotional  costs for the
Building;

                  (xii)  Payments to a company or other entity  affiliated  with
Landlord  or  Landlord's  management  company  (including,  without  limitation,
payments for real estate management services) to the extent such payments exceed
the amount that would have been paid to independent  third parties for goods and
services of like kind;

                  (xiii) Costs and expenses in  connection  with the  abatement,
encapsulation  or  removal  of  Hazardous   Materials,   including  asbestos  or
asbestos-containing materials;

                  (xiv) Costs and expenses  incurred to remedy conditions in the
Building that violate  requirements  of law,  rules,  ordinances or  regulations
which took effect or were enacted prior to the date of this Lease;

                  (xv)  Costs and  expenses  attributable  to the  negligence or
willful  misconduct  of Landlord,  its  employees or agents,  or any  management
company at the Building;

                  (xvi) Equipment leases for equipment leased in lieu of capital
expenditures that would have been excluded pursuant to this Section 7.3; and

                  (xvii) Costs of any capital expenditures, except to the extent
such  capital  expenditures  may be included in Operating  Expenses  pursuant to
clause 7.2(xi), above.

                  In addition, in the calculation of any costs or expenses under
this  Article 7, it is  expressly  understood  that no cost or expense  shall be
charged more than once.

         7.4 ESTIMATED  PAYMENTS.  Landlord  shall submit to Tenant,  before the
beginning  of each  Calendar  Year, a statement  of  Landlord's  estimate of the
Operating  Expenses  payable by Tenant during such Calendar Year. In addition to
the Base Rent,  Tenant  shall pay to Landlord on or before the first day of each
month  during  such  Calendar  Year an amount  equal to  one-twelfth  (1/12) the
estimated  Operating  Expenses  payable by Tenant for such  Calendar Year as set
forth in Landlord's  statement.  If Landlord  fails to give Tenant notice of its
estimated  payments due under this Section for any  Calendar  Year,  then Tenant
shall continue making monthly estimated payments in accordance with the estimate
for the previous  Calendar  Year until a new  estimate is provided.  If Landlord
determines that, because of unexpected  increases in Operating Expenses or other
reasons, Landlord's estimate of the Operating Expense was too low, then Landlord
shall have the right to give a new statement of the estimated Operating Expenses
due from Tenant for such Calendar Year or the balance thereof and to bill Tenant
for any deficiency  which may have accrued during such Calendar Year, and Tenant
shall thereafter pay monthly estimated payments based on such new statement.

         7.5 ACTUAL  OPERATING  EXPENSES.  Within one hundred  twenty (120) days

<PAGE>

after the end of each Calendar Year, Landlord shall submit a statement to Tenant
showing  the actual  Operating  Expenses  for such  Calendar  Year and  Tenant's
Proportionate  Share of the amount by which such Operating  Expenses  exceed the
Operating  Expenses  during the Base Year.  If for any Calendar  Year,  Tenant's
estimated monthly payments exceed Tenant's  Proportionate Share of the amount by
which the actual Operating  Expenses for such Calendar Year exceed the Operating
Expenses  during the Base Year,  then  Landlord  shall either pay such amount to
Tenant within thirty (30) days after the date of such statement,  or give Tenant
a credit in the amount of the overpayment  toward Tenant's next monthly payments
of estimated  Operating  Expenses  and/or Base Rent.  If for any  Calendar  Year
Tenant's estimated monthly payments are less than Tenant's  Proportionate  Share
of the amount by which the accrual  Operating  Expenses for such  Calendar  Year
exceed the Operating  Expenses  during the Base Year,  then Tenant shall pay the
total  amount of such  deficiency  to  Landlord  within  thirty  (30) days after
receipt of the statement from Landlord. Landlord's and Tenant's obligations with
respect to any overpayment or  underpayment of Operating  Expenses shall survive
the expiration or termination of this Lease.

         7.6 TENANT'S  RIGHT TO AUDIT.  In the event  Tenant  shall  dispute the
amount set forth in Landlord's  statement of actual Operating  Expenses,  Tenant
shall have the right,  not later than sixty (60) days following  receipt of such
statement,  to cause  Landlord's books and records with respect to the preceding
Calendar  Year to be audited  by Tenant or by an  independent  Certified  Public
Accountant  mutually  acceptable to Landlord and Tenant, in which event Landlord
will make all pertinent books and records for the subject Calendar Year (and, if
applicable, Base Year) available to Tenant or its Auditor at Landlord's offices,
or another mutually acceptable location within the Washington, D.C. metropolitan
area.  Such audit  shall  occur  upon not less than five (5) days prior  written
notice to Landlord,  at Landlord's  place of business or the actual  location of
Landlord's books and records if different from Landlord's place of business (but
in all events within the Washington,  D.C. metropolitan area , during Landlord's
normal  business  hours.  The amounts  payable under this Article by Landlord to
Tenant or by  Tenant to  Landlord,  as the case may be,  shall be  appropriately
adjusted on the basis of such audit, provided that, if Tenant performs the audit
on its own, or does not use an independent  certified public accountant mutually
and reasonably  acceptable to both Landlord and Tenant,  Landlord shall have the
right to dispute  the  results of any such  audit,  as more fully  provided  for
below.  If such audit  discloses a liability  for further  refund by Landlord to
Tenant in excess of [***] of the  payments  previously  made by Tenant  for such
Calendar  Year,  the cost of such audit shall be borne by Landlord and shall not
be considered as an Operating Expense for purposes of this Lease; otherwise, the
cost of such audit shall be born by Tenant. Notwithstanding the foregoing, in no
event shall  Landlord's  cost for such audit exceed  [***].  If Tenant shall not
request an audit in accordance  with the provisions of this Section within sixty
(60) days of receipt  of  Landlord's  statement  of actual  Operating  Expenses,
Tenant  shall have no further  right to audit or modify  Operating  Expenses for
such Calendar Year (or, if  applicable,  the Base Year).  In the event  Landlord
wishes to  dispute  the  results  of any audit  conducted  by Tenant  under this
Section 7.6,  Landlord shall so notify Tenant in writing within thirty (30) days
after its receipt  thereof,  and, if the parties  cannot  resolve  such  dispute
voluntarily  within  thirty (30) days after such notice,  the matters in dispute
will be submitted  to an  independent  certified  public  accountant  reasonably
acceptable to Landlord and Tenant (the "Independent  CPA") , with Tenant to pay,
or Landlord to retain, the amount in dispute,  if any, pending the resolution of
such dispute by the Independent  CPA, and with the  non-prevailing  party to pay
all fees and costs  charged by the  Independent  CPA to conduct  such review and
resolve such dispute (and, if Tenant  prevails,  with Landlord to pay the amount
which was  overpaid by Tenant,  such fees and costs as  aforesaid,  and interest
thereon at the Interest  Rate from the date such amount was paid by Tenant until
the date such amount was reimbursed by Landlord).

         7.7 FURTHER ADJUSTMENT. In the event Landlord shall furnish any utility
or service  which is included in the  definition  of Operating  Expenses to less
than ninety-five  percent (95%) of the rentable area of the Building because (i)
the  average  occupancy  level of the  Building  for the Base  Year  and/or  any
subsequent  calendar  year  was not  ninety-five  percent  (95%) or more of full
occupancy,  (ii) any such  utility or service is not  required by or provided to
one or more of the tenants or  occupants  of the  Building,  and such  tenant(s)
is(are) not required to contribute  its(their)  proportionate  share thereof, or
(iii) any tenant or occupant is itself  obtaining or providing  any such utility
or services  directly,  then the Operating Expenses for such year (including the
Base Year)  shall be  adjusted to include all  additional  costs,  expenses  and
disbursements that Landlord  reasonably  determines would have been incurred had
the Building been ninety-five percent (95%) occupied during the year in question
and such  utilities  and services  provided to all  tenants.  The intent of this
Section 7.7 is to ensure that the  reimbursement  of all  Operating  Expenses is
fair and equitably  allocated  among the tenants  receiving  such  utilities and
services.  In the calculation of Operating Expenses hereunder,  no expense shall
be charged more than once.

<PAGE>

                                    ARTICLE 8
                                      TAXES

         8.1  GENERALLY.  Tenant shall pay to Landlord  throughout  the Term, as
Additional Rent,  Tenant's  Proportionate  Share of the amount by which the Real
Estate Taxes during each  Calendar  Year exceed the Real Estate Taxes during the
Base Year. In the event that the  Commencement  Date or the Expiration  Date are
other than the first day of a Calendar Year, the Tenant's Proportionate Share of
Real Estate  Taxes shall be adjusted to reflect the actual  period of  occupancy
during  the  Calendar  Year.  "Real  Estate  Taxes"  shall  mean all  taxes  and
assessments,  including  but not  limited to,  general or  special,  ordinary or
extraordinary,  foreseen  or  unforeseen,  assessed,  levied or  imposed  by any
governmental  authority  upon the Building  and the Land and upon the  fixtures,
machinery,  equipment or systems in, upon or used in connection  with any of the
foregoing  (exclusive of personal property owned by any other tenant or occupant
of the  Building),  and  the  rental,  revenue  or  receipts  derived  therefrom
(exclusive of income taxes and/or  franchise  taxes on such  income),  under the
current  or any  future  taxation  or  assessment  system  or  modification  of,
supplement  to, or  substitute  for such  system.  Real Estate  Taxes also shall
include special  assessments  which are in the nature of or in substitution  for
real estate taxes, including,  without limitation, road improvement assessments,
special use area assessments and school district assessments. If at any time the
method of taxation  prevailing  at the Date of Lease shall be altered so that in
lieu of,  as a  substitute  for or in  addition  to the whole or any part of the
taxes now  levied or  assessed,  there  shall be  levied  or  assessed  a tax of
whatever  nature  which is assessed  against  property  owners or  operators  of
commercial  real estate  projects  in respect of property or the income  derived
therefrom (but which is not an income tax or franchise tax), then the same shall
be included as Real Estate Taxes hereunder. Landlord shall charge Tenant for its
Proportionate  Share of Real  Estate  Taxes in  accordance  with the  procedures
established under Sections 7.4 and 7.5 for payment of Operating Expenses.

         8.2 CONTESTING TAXES. Within thirty (30) days after receipt of a notice
of  assessment  with  respect to the Real Estate Taxes for the  Building,  which
assessment relates to a period entirely within the Term of this Lease,  Landlord
shall deliver to Tenant a copy of the same,  together with written  notice as to
whether Landlord intends to contest such  assessment.  If Landlord  contests any
such assessment, Tenant shall cooperate, and shall have the right to participate
at its expense,  in such  contest (and the  reasonable  costs  thereof  shall be
included  within the definition of Real Estate Taxes  herein).  If Landlord does
not elect to  contest  such  assessment,  then  Tenant  shall  have the right to
contest, by appropriate administrative or legal proceedings, such assessment (or
the Real Estate Taxes  applicable  to the  Building),  as long as the period for
which such Real  Estate  Taxes are being  contested  does not extend  beyond the
Expiration Date of this Lease,  and provided further that: (i) Tenant shall give
Landlord  prior  written  notice of its  intention  to contest  such tax and the
identity of its tax counsel or consultant,  which shall be subject to Landlord's
approval  (which shall not be  unreasonably  withheld,  conditioned or delayed);
(ii) such contest  shall not be permitted to proceed if it could cause or result
in a sale or  foreclosure  of such  Building,  or impose  any civil or  criminal
penalties  upon  Landlord;  (iii)  Tenant shall timely pay all Real Estate Taxes
necessary to pursue such contest if and to the extent  payment is necessary  for
the  prosecution  of such  protest or  contest;  (iv)  Tenant  shall  diligently
prosecute  such  contest,  and keep  Landlord  informed on a regular  basis with
respect  thereto  (including  provision  of copies to  Landlord  of all  written
filings made or received by Tenant  therein);  (v) Tenant shall pay any interest
or penalties  with respect to the tax protested or contested,  (vi) Tenant shall
not cancel,  discontinue or settle such  proceedings  without  Landlord's  prior
written  consent  (which  shall not be  unreasonably  withheld,  conditioned  or
delayed), and (vii) Tenant shall be responsible for (and shall pay as Additional
Rent  hereunder)  all  increased  Real Estate Taxes  resulting  from any adverse
decision or action taken as a result of such tax contest.  If as a result of any
such  challenge,  a tax refund is made to Landlord,  then Tenant's  Share of the
difference  between the amount of such refund less the expenses of the challenge
shall be paid to Tenant  within  thirty  (30) days  after the date  received  by
Landlord (or Tenant shall receive a credit toward Tenant's next monthly payments
of estimated Operating Expenses and/or Base Rent in such amount).

                                    ARTICLE 9
                                     PARKING

         9.1 PARKING SPACES. During the Term, Tenant shall have the right to use
the Parking Space  Allocation for its  employees,  agents,  business  guests and
invitees.  Tenant's  Parking Space  Allocation  shall consist of unreserved  and

<PAGE>

non-exclusive  parking spaces available in the Parking Facilities.  Tenant shall
not  overburden the Parking  Facilities,  and any usage in excess of the Parking
Space  Allocation  may, at  Landlord's  election,  be deemed to  overburden  the
Parking  Facilities,  provided Landlord agrees that it will forbear on asserting
any claim of Default against Tenant for overburdening the Parking  Facilities as
long as no other tenant or occupant of the Building is  complaining  to Landlord
with regard to the excessive or  disproportionate  use of the Parking Facilities
by Tenant and its employees, agents, business guests and invitees.

         9.2 CHANGES TO PARKING FACILITIES.  Landlord shall have the right, from
time to time,  without Tenant's consent,  to change,  alter, add to, temporarily
close or otherwise affect the Parking Facilities in such manner as Landlord,  in
its sole discretion,  deems appropriate including, without limitation, the right
to  designate  reserved  spaces  available  only for use by one or more  tenants
(however,  in such event, those parking spaces shall still be deemed Common Area
for the purpose of the  definition  of  Operating  Expenses),  PROVIDED  that in
conducting any  modifications or closures to the Parking  Facilities (other than
temporary closings for parking lot maintenance or repair, or other improvements,
modifications or expansions to the Building and/or Common Areas which impact the
Parking Facilities,  which will be conducted so as to minimize interference with
Tenant's  use and  enjoyment  of the Parking  Spaces),  Landlord  shall  provide
alternative  parking  facilities  reasonably  acceptable  to Tenant so as not to
decrease  the number of parking  spaces  available  to Tenant  below the Parking
Space  Allocation.  In addition,  if Landlord  exercises  its right to designate
reserved  parking spaces for a tenant under any lease of the Building,  Landlord
agrees to designate  for the benefit of Tenant a  comparable  number of reserved
parking  spaces  (and no  reservation  of  reserved  parking  for any  tenant or
occupant of the Building  shall be construed to reduce  Tenant's  Parking  Space
Allocation).

                                   ARTICLE 10
                                       USE

         Tenant  shall  occupy the Premises  solely for the  Permitted  Use. The
Premises  shall  not be used for any other  purpose  without  the prior  written
consent of Landlord. Subject to Landlord's obligation to deliver the Premises to
Tenant in compliance with applicable laws, rules and regulations  (except as the
same is limited under EXHIBIT B), Tenant shall comply, at Tenant's expense, with
(i) all  present  and future  laws,  ordinances,  regulations  and orders of the
United States of America,  the  Commonwealth of Virginia and any other public or
quasi-public federal, state or local authority having jurisdiction over Tenant's
use,  occupancy  and  operations  within the Premises,  and (ii) any  reasonable
requests of Mortgagee or any insurance company  providing  coverage with respect
to  the  Premises.   The  foregoing   notwithstanding,   Landlord  shall  remain
responsible  for any  improvements  required by the Americans with  Disabilities
Act, any by applicable  life,  fire and safety codes or similar laws,  rules and
regulations,  except (i) Tenant shall be responsible for all of the foregoing to
the extent  arising out of (A) Tenant's  specific use of the  Premises,  (B) the
failure of Tenant Improvement designed by Tenant's architect pursuant to EXHIBIT
B to so  comply,  unless  (1)  such  failure  is the  result  of  deviations  in
construction  from Approved Plans,  (2) the applicable item within such Approved
Plans would have been in compliance  with such laws,  codes and  regulations but
for such  deviation(s),  and (3) such deviation is itself NOT due to the acts or
omissions of a general  contractor,  subcontractor  or other party designated or
selected by Tenant,  and/or (C) any  Alterations  to the Premises  made by or on
behalf of Tenant, and (ii) Landlord shall have the right to include the expenses
associated  with any such  improvements  as  Operating  Expenses  to the  extent
permitted  under  Sections  7.2  and  7.3  of  the  Lease  (and  subject  to the
limitations set forth  therein).  Tenant shall not use or occupy the Premises in
any  manner  that is  unlawful  or  dangerous  or that shall  constitute  waste,
unreasonable  annoyance  or a nuisance to  Landlord or the other  tenants of the
Building.  Tenant shall not use, store or dispose of any  hazardous,  dangerous,
inflammable,  toxic or explosive materials on the Premises, other than Permitted
Materials (as defined in, and solely to the extent allowed under,  Article 26 of
this Lease).

                                   ARTICLE 11
                            ASSIGNMENT AND SUBLETTING

         11.1 CONSENT REQUIRED.  Subject to the terms of this Section 11, Tenant
shall not assign, encumber,  mortgage, pledge, license, hypothecate or otherwise
transfer  the  Premises  or  this  Lease,  or  sublease  all or any  part of the
Premises, or permit the use or occupancy of the Premises by any party other than
Tenant,  without  the prior  written  consent of  Landlord,  which  shall not be
unreasonably withheld as more fully set forth below.

<PAGE>

         11.2     PROCEDURE.

                  11.2.1  Tenant  must  request  Landlord's  consent to any such
assignment  or sublease in writing at least ten (10)  business days prior to the
commencement date of the proposed sublease or assignment,  which written request
(a  "Proposal  Notice")  must  include (1) the name and address of the  proposed
assignee  or  subtenant,  (2) the nature and  character  of the  business of the
proposed assignee or subtenant,  (3) financial information  (including financial
statements) of the proposed  assignee or subtenant,  (4) the proposed  effective
date of the  assignment  or  sublease,  which  shall be not  less  than ten (10)
business  days  thereafter,  and (5) an executed  term sheet or letter of intent
setting forth all material  terms and  conditions of the proposed  assignment or
sublease  (the  "Term   Sheet").   Tenant  shall  also  provide  any  additional
information  Landlord  reasonably requests regarding such proposed assignment or
subletting.  Within ten (10)  business  days after  Landlord  receives  Tenant's
Proposal  Notice (with all required  information  and including the Term Sheet),
but subject to Section 11.5,  below,  to the extent  applicable,  Landlord shall
have  the  option  (i) to grant  its  consent  to such  proposed  assignment  or
subletting,  or  (ii)  to deny  its  consent  to  such  proposed  assignment  or
subletting on a reasonable basis. If Landlord does not exercise one of the above
options (or the termination  right set forth in Section 11.5,  below) within ten
(10) business days after Landlord  receives such Proposal Notice and Term Sheet,
then Tenant may assign or sublease  the  Premises  upon the terms  stated in the
Proposal  Notice (and Term Sheet)  provided  that Tenant shall provide a copy of
the executed  assignment or sublease to Landlord  prior to the effective date of
such assignment or sublease in order for Landlord to confirm that there has been
no material  deviation  between the terms set forth in the  Proposal  Notice and
Term Sheet and the terms set forth in the final executed sublease or assignment,
it being agreed that any change in the net  effective  rent payable  pursuant to
the  assignment or sublease as originally  set forth in the Proposal  Notice and
Term Sheet  shall be deemed to be  material  (and if there is any such  material
deviation,  the delivery of the  executed  sublease or  assignment  by Tenant to
Landlord shall be deemed to constitute the delivery of a new Proposal  Notice by
Tenant to Landlord,  subject to Landlord's  approval and/or recapture rights, as
applicable under this Section 11.2).

                  11.2.2 Section 11.5,  below, to the contrary  notwithstanding,
Tenant shall have the right to enter into Short Term  Subleases  as  hereinafter
defined without  triggering  Landlord's right of termination under Section 11.5,
but subject to Landlord's  reasonable consent as determined  pursuant to Section
11.2.3,  below.  The term "Short Term  Sublease"  shall mean any sublease  which
meets all of the following tests: (A) when aggregated with the square footage of
all other subleases entered into pursuant to this Lease, the subleased  premises
is less than [***] square feet of rentable area, (B) such sublease has a term of
[***] years or less, and (C) such sublease does not end during the last [***] of
the Term. In addition to the foregoing rights relating to Short-Term  Subleases,
Tenant  will also have the right to enter into  subleases  (hereinafter  "Vendor
Subleases")  for up to [***] square feet in the aggregate,  in each case without
triggering  Landlord's right of termination under Section 11.5 and/or Landlord's
rights under Section  11.3.1,  provided (a) such  subleases  shall be limited to
individual  offices  which are not  separately  demised  from the balance of the
Premises, (b) such subleases are entered into with vendors,  suppliers, or other
companies  with whom Tenant has  ongoing  contractual  relations  which make the
sublease in question reasonably  necessary or appropriate for the fulfillment of
a  business   purpose   specifically   related  to  the   conduct  of   Tenant's
telecommunications  business, AND (c) the proposed subtenant: (1) is solvent and
is otherwise able to meet its financial and other obligations under its sublease
and with  respect to its sublease  premises,  and (2) does not have a history of
landlord/tenant, debtor/creditor or other contractual problems (such as, but not
limited to, defaults, evictions,  enforcement litigation or other disputes) with
Landlord, other landlords and/or creditors or other contracting parties.

                  11.2.3 Without  limitation,  it shall not be unreasonable  for
Landlord to deny its consent to any proposed assignment or sublease which is not
a Vendor  Sublease  if: (1) the  proposed  assignee or sublessee is in financial
difficulty,  is insolvent, or it otherwise appears that the proposed assignee or
subtenant may be unable to meet its financial and other  obligations  under this
Lease after such assignment or sublease;  (2) the proposed assignee or subtenant
proposes  to use  the  Premises  for a  purpose  which  is not a  Permitted  Use
hereunder,  or which involves the provision of maintenance or services in excess
of that normally provided to office tenants generally; (3) the proposed assignee
or  subtenant  has  a  history  of  landlord/tenant,  debtor/creditor  or  other
contractual problems (such

<PAGE>

as, but not limited to,  defaults,  evictions,  enforcement  litigation or other
disputes) with Landlord,  other landlords and/or creditors or other  contracting
parties;  (4) the proposed  assignee or subtenant is an existing tenant,  or the
affiliate of an existing  tenant,  in any building owned or operated by Landlord
or any affiliate of Landlord;  (5) the proposed assignment or sublease has a net
effective  rental  rate  which is less than  [***] of the then fair  market  net
effective rental rate applicable to comparable space in comparable  buildings in
the Reston, Virginia submarket; and/or (6) the proposed assignee or subtenant is
entitled to, or otherwise enjoys, sovereign or diplomatic immunity.

         11.3 CONDITIONS.  Any subleases and/or  assignments  hereunder are also
subject to all of the following terms and conditions:

                  11.3.1 If  Landlord  approves  an  assignment  or  sublease as
herein provided, Tenant shall pay to Landlord, as additional rent due under this
Lease,  (i)  in  the  case  of  an  assignment, all sums received  by  Tenant in
consideration  of such  assignment  (exclusive  of sums paid to Tenant for other
assets  transferred  by Tenant as part of such  transaction),  calculated  after
Tenant has recovered in full from such consideration its "Transaction  Expenses"
(as hereafter defined),  and (ii) in the case of a sublease, the amount, if any,
by which  the  rent,  any  additional  rent and any other  sums  payable  by the
subtenant to Tenant under such  sublease,  exceeds that portion of the Base Rent
plus  Additional  Rent  payable by Tenant  hereunder  which is  allocable to the
portion of the Premises which is the subject of such sublease,  calculated after
Tenant has recovered in full its Transaction  Expenses from such net amount. The
term "Transaction  Expenses" shall mean all reasonable and actual  out-of-pocket
expenses  incurred by Tenant in  procuring  such  assignment  or  sublease.  The
foregoing  payments shall be made on not less than a monthly basis by Tenant (in
the case of  subleases)  and in all cases  within ten (10)  business  days after
Tenant receives the applicable consideration from the assignee or subtenant.

                  11.3.2  No  consent  to  any   assignment  or  sublease  shall
constitute  a  further  waiver  of the  provisions  of  this  section,  and  all
subsequent  assignments  or  subleases  may be made only with the prior  written
consent of  Landlord.  In no event shall any consent by Landlord be construed to
permit reassignment or re-subletting by a permitted assignee or sublessee.

                  11.3.3 The assignee  under any  assignment of this Lease shall
be fully (and, at landlord's option, directly) liable for all of the obligations
of "Tenant" under this Lease,  on a joint and several basis with Tenant.  Tenant
shall  nevertheless  remain fully liable to Landlord for all Lease  obligations,
including those accruing after the effective date of such assignment.

                  11.3.4  Any  sublease  or  assignment  shall be subject to the
condition that the sublessee or assignee thereunder shall be bound by all of the
terms,  covenants  and  conditions  of this  Lease  (in the case of a  sublease,
insofar as such terms,  covenants  and  conditions  relate to the portion of the
Premises  subleased  and/or  the  operations  and  conduct  of  business  by the
sublessee).

                  11.3.5  Without  limitation,  any and all  guaranties  of this
Lease shall be unaffected by such sublease and  assignment,  and shall remain in
full force and effect for all purposes.

                  11.3.6 Any  assignment or sublease  without  Landlord's  prior
written  consent  shall be void,  and  shall,  at the  option  of the  Landlord,
constitute a default under this Lease.

                  11.3.7 Tenant  shall  pay  to Landlord a processing fee of Two
Hundred Fifty and No/100 Dollars  ($250.00),  which shall accompany any proposed
assignment or sublease delivered by Tenant to Landlord, and which processing fee
shall be in addition to Landlord's  reasonable  attorneys fees and out-of-pocket
expenses  incurred in  connection  with  Landlord's  review of such  sublease or
assignment (if any), which shall also be reimbursed by Tenant.

<PAGE>

         11.4     AFFILIATED ENTITY; SALE OF BUSINESS.

                  11.4.1 Notwithstanding anything to the contrary in this Lease,
so long as such transfer is not  effectuated  as part of a transaction or series
of  transfers  orchestrated  in order to  effect a  transfer  of this  Lease (or
Tenant's interest herein) in isolation to Tenant's other leasehold interests and
assets,  Tenant may assign  this Lease or sublet all or part of the  Premises to
any entity (i) which controls or is controlled by Tenant, or (ii) which is under
common control with Tenant, or (iii) which purchases all or substantially all of
the assets of Tenant,  or (iv) which purchases all or  substantially  all of the
stock of Tenant or (v) which  merges with Tenant  pursuant to a valid  statutory
merger; PROVIDED, that (1) the assignee or sublessee is solvent and is otherwise
financially able to meet all of its obligations under the proposed assignment or
sublease,  and (2) in such event,  (a) except in cases of statutory  merger,  in
which  case the  surviving  entity in the  merger  shall be liable as the Tenant
under this Lease,  Tenant shall continue to remain fully liable under the Lease,
on a joint and  several  basis with the  assignee  or acquiror of such assets or
stock,  (b) the terms of any  guaranty of this Lease shall  remain in full force
and effect, unmodified, and (c) following such sublease or assignment, Tenant or
such  assignee,  as the case may be,  shall  continue  to comply with all of its
obligations under this Lease, including with respect to its Permitted Use of the
Premises.

                  11.4.2  Tenant  shall be  required  to give  Landlord at least
twenty (20) days written notice in advance of any sublease or assignment  within
the scope of Section 11.4.1, above. Any other transfer of fifty percent (50%) or
more of the ownership  interests  (including,  without  limitation,  partnership
interests  or stock) in Tenant or of operating  control over Tenant  (whether by
management  agreement,  stock sale or other means) shall be deemed to constitute
an  assignment  of this  Lease,  and shall be subject to  Landlord's  consent as
aforesaid,  PROVIDED  that this  sentence will not apply for so long as Tenant's
stock is listed for sale as a publicly traded security on a national  securities
exchange.

                  11.4.3  Notwithstanding the last sentence of Section 11.4.2 to
the  contrary,  Landlord  agrees  that  the  offer  and sale by  Tenant  (or any
stockholder  of  Tenant)  of any stock  pursuant  to an  effective  registration
statement  filed pursuant to the  Securities Act of 1933  (including any initial
public  offering  of  registered  stock of the  Tenant)  or  pursuant  to and in
accordance with the securities laws of any foreign  country  governing  publicly
traded  companies  and not in violation of U.S.  law,  shall not  constitute  an
assignment  of this  Lease,  and shall not  require  the  consent or approval of
Landlord.

                  11.4.4 Tenant shall not transfer all or  substantially  all of
its  assets to any person or entity  unless  either (i) this Lease is one of the
assets so transferred to such other person or entity, and the transferee assumes
in writing, for Landlord's benefit, the obligations of Tenant accruing hereunder
from and after the  effective  date of the transfer,  or (ii) the  transferee(s)
thereof  otherwise  delivers  to  Landlord  a  written  assumption  of  Tenant's
obligations hereunder.

         11.5.  RIGHT OF  TERMINATION.  Except for any  sublease  or  assignment
permitted  pursuant  to  Section  11.4,  above,  in the event of (i) a  proposed
assignment of this Lease, or (ii) a proposed  sublease which is not a Short Term
Sublease  or a Vendor  Sublease,  Landlord  shall have the  right,  by notice to
Tenant  delivered  within ten (10)  business  days after  Landlord's  receipt of
Tenant's  Proposal  Notice  together  with the executed  assignment  or sublease
instrument  (and in lieu of the  granting or denial of consent  provided  for in
Section 11.2,  above), to terminate this Lease as to all of the Premises (in the
event of an assignment) or as to the proposed  subleased portion of the Premises
only (in the event of a sublease),  in each case for the balance of the Term. In
the event  Landlord  shall elect to terminate  this Lease in  connection  with a
proposed  assignment or sublease of this Lease as provided  above in whole or in
part (as the case may be):  (a) this Lease and the term hereof  shall  terminate
(either as to the Premises as a whole,  or only as to the portion  thereof which
Tenant is proposing to sublease,  as the case may be) as of the later of (i) the
proposed effective date of such assignment or sublease, as set forth in Tenant's
Proposal Notice, or (ii) ten (10) business days after the date Landlord received
Tenant's  Proposal  Notice  together  with the executed  assignment  or sublease
instrument;  (b) Tenant shall be released from all liability under the Lease (as
to  the  Premises  as a  whole,  in  the  case  of an  assignment,  or as to the
terminated  portion of the Premises  only, in the case of a partial  termination
due to sublease) with respect to the period after the date of termination (other
than  obligations  and  indemnities  of Tenant which accrued with respect to the
applicable  portion  of  the  Premises  prior  to the  effective  date  of  such
termination,  which  obligations  shall  expressly  survive such  termination or
partial termination of this Lease); (c) all Base Rent, additional rent and other
charges  shall be prorated to the date of such  termination,  and  appropriately

<PAGE>

adjusted if there is only a partial termination; (d) upon such termination date,
Tenant  shall  surrender  the Premises (or the  applicable  portion  thereof) to
Landlord in accordance with Section 26 hereof;  and (e) in the case of a partial
termination  of this Lease,  Landlord  shall have the obligation to separate the
portion of the  Premises  being  terminated  from the  balance of the  Premises,
including the erection of a demising wall and, to the extent necessary under the
circumstances  (and  reasonably  practicable  given  the  configuration  of  the
applicable portion of the Premises),  the separation of any applicable  Building
Systems.

                                   ARTICLE 12
                             MAINTENANCE AND REPAIR

         12.1  LANDLORD'S  OBLIGATION.  Landlord shall keep and maintain in good
repair and working order,  consistent  with standards  applicable to other first
class office  buildings in the Reston,  Virginia  submarket,  the Building,  the
Common Area and the  equipment  within and serving the Premises and the Building
(excluding  Tenant's  leasehold  improvements in the Premises) that are required
for the normal  maintenance and operation of the Premises and the Building.  The
cost of such  maintenance and repairs to the Building,  and said equipment shall
be included in the Operating  Expenses and paid by Tenant as provided in Article
7 herein. Tenant shall immediately give Landlord written notice of any defect or
need  for  repairs.  After  such  notice,   Landlord  shall  have  a  reasonable
opportunity to repair or cure such defect.

         12.2 TENANT'S  OBLIGATION.  Subject to Article 21 of this Lease, Tenant
shall, at its own expense,  maintain all of Tenant's  leasehold  improvements in
the Premises and other real and  personal  property  within the Premises in good
condition, promptly making all necessary repairs and replacements.  Tenant shall
repair at its expense any and all damage caused by the negligent or willful acts
or omissions of Tenant or Tenant's  Agents to the Building,  the Common Area, or
the Premises, including equipment within and serving the Building, ordinary wear
and tear excepted. Notwithstanding the foregoing, Tenant shall bear the cost of,
but shall not itself perform without Landlord's prior consent,  any such repairs
which would affect the Building's  structure or mechanical or electrical systems
or which would be visible from the exterior of the Building or from any interior
Common Area of the Building.  Where Landlord performs such repairs, Tenant shall
pay to  Landlord  within  thirty  (30) days  after  Landlord's  demand all costs
incurred in connection  therewith;  and if such amounts are not paid within such
thirty (30) day period,  the same shall bear  interest at the Interest Rate from
the date originally demanded by Landlord until the date actually paid by Tenant.
Without the prior  written  consent of the Landlord,  or as expressly  permitted
pursuant to Section 29.2 of this Lease, Tenant shall not have access to the roof
of the Building for any purpose whatsoever.

         12.3  LANDLORD'S  RIGHT TO MAINTAIN OR REPAIR.  If, within fifteen (15)
days following notice to Tenant (or such shorter period as is appropriate in the
event of an Emergency), Tenant fails to commence to repair or replace any damage
to the  Premises  or  Building  which is Tenant's  obligation  to  perform,  and
diligently  pursue timely  completion of such repair and  replacement,  Landlord
may, at its option, cause all required  maintenance,  repairs or replacements to
be made.  Tenant shall  promptly pay Landlord all costs  incurred in  connection
therewith  plus  interest  thereon at the Interest  Rate from the due date until
paid.

                                   ARTICLE 13
                        INITIAL CONSTRUCTION; ALTERATIONS

         13.1  INITIAL   CONSTRUCTION.   Landlord  and  Tenant  agree  that  the
construction of the Tenant Improvements (as defined in Exhibit B attached hereto
and made a part  hereof)  and other  initial  construction  with  respect to the
Premises shall be performed in accordance with Exhibit B.

         13.2  ALTERATIONS.  Tenant  shall  not make or permit  any  Alterations
without  the  prior  written  consent  of  Landlord.  Landlord  may  impose  any
reasonable  conditions  to  its  consent,  including,  without  limitation,  (i)
delivery  to Landlord of written and  unconditional  waivers of  mechanic's  and
materialman's liens as to the Premises,  the Building and the Land for all work,

<PAGE>

labor and services to be performed and materials to be furnished,  signed by all
contractors,  subcontractors,  materialmen  and  laborers  participating  in the
Alterations,  (ii) prior approval of the plans and  specifications  and Tenant's
contractor(s)  with respect to the Alterations,  (iii) supervision by Landlord's
representative  at  Tenant's  expense of the  Alterations  and (iv)  delivery to
Landlord of payment and  performance  bonds  naming  Landlord  and  Mortgagee as
obligees.  The Alterations  shall conform to the  requirements of Landlord's and
Tenant's  insurers  and of the  Federal,  state  and  local  governments  having
jurisdiction over the Premises,  shall be performed in accordance with the terms
and provisions of this Lease in a good and workmanlike  manner befitting a first
class  office  building  and shall not  adversely  affect the value,  utility or
character  of the  Premises.  If the  Alterations  are not  performed  as herein
required,  Landlord  shall have the right,  at  Landlord's  option,  to halt any
further  Alterations,  or to require Tenant to perform the Alterations as herein
required or to require  Tenant to return the  Premises to its  condition  before
such  Alterations.   Notwithstanding   the  foregoing,   if  any  mechanic's  or
materialmen's  lien is filed against the Premises,  the Building or the Land for
work done or claimed to have been done for,  or  materials  claimed to have been
furnished to or for the benefit of,  Tenant,  such lien shall be  discharged  of
record by Tenant  within ten (10) days by the  payment  thereof or the filing of
any bond  required  by law.  If Tenant  shall fail to  discharge  any such lien,
Landlord may (but shall not be obligated  to)  discharge  the same,  the cost of
which shall be paid by Tenant within three (3) days of demand by Landlord.  Such
discharge  by  Landlord  shall not be deemed to waive or release  the default of
Tenant  in  not  discharging  the  same.   Neither  Landlord's  consent  to  the
Alterations  nor anything  contained  herein in this Lease shall be deemed to be
the  agreement  or consent of  Landlord  to subject  Landlord's  interest in the
Premises,  the Building or the Land to any  mechanic's  or  materialmen's  liens
which may be filed in respect of the Alterations.

         13.3 REMOVAL OF ALTERATIONS.  If Landlord fails to notify Tenant at the
time of its approval of any proposed  Alterations  that such  Alteration must be
removed upon expiration or earlier  termination of this Lease elects,  then such
Alteration  shall be surrendered  with the Premises at the  Expiration  Date (or
earlier  termination  of this  Lease) and shall  thereupon  be the  property  of
Landlord.  If Landlord notifies Tenant at the time of such approval that it will
require the removal of all or part of the  Alterations  and Tenant proceeds with
such Alterations, Tenant shall, at its expense, remove such Alterations from the
Premises,  repair  any damage to the  Premises  or the  Building  caused by such
removal,  and restore the affected  portion of the Premises to its  pre-existing
condition upon the Expiration  Date (or earlier  termination of this Lease).  If
Tenant thereupon fails to remove such Alterations upon the expiration or earlier
termination  of this Lease (or to conduct  such repairs and  restoration  at the
time of such removal),  then Landlord may (but shall not be obligated to) remove
the same, and make any necessary  repairs and restoration,  and the cost of such
removal  and  restoration,  and the  repair  of any  damage  caused by the same,
together  with any and all  damages  which  Landlord  may suffer and  sustain by
reason of the  failure of Tenant to remove the same,  shall be charged to Tenant
and paid upon demand.

         13.4 LANDLORD  ALTERATIONS.  Landlord  shall have no obligation to make
any Alterations in or to the Premises, the Building, the Common Area or the Land
except as specifically  provided in the Work Agreement or to the extent required
to keep the  Building,  Common Area and Land in the  condition  required by this
Lease.  Landlord  hereby  reserves  the  right,  from  time  to  time,  to  make
Alterations to the Building,  change the Building  dimensions,  erect additional
stories thereon and attach other buildings and structures thereto,  and to erect
such  scaffolding and other aids to construction as Landlord deems  appropriate,
and,  provided that such activities do not unreasonably  interfere with Tenant's
use and  enjoyment  of the Premises or  materially  reduce its rights under this
Lease, no such Alterations,  changes,  construction or erection shall constitute
an eviction,  constructive or otherwise,  or permit Tenant any abatement of Rent
or claim.

                                   ARTICLE 14
                                      SIGNS

         14.1 GENERAL SIGN PROVISION.  Except as expressly  permitted hereby, no
sign,  advertisement or notice shall be inscribed,  painted,  affixed, placed or
otherwise  displayed  by  Tenant on any part of the Land or the  outside  or the
inside  (including,  without  limitation,  the  windows) of the  Building or the
Premises.  Any permitted  signs shall be installed and maintained by Landlord at
Tenant's sole expense.  All signage shall be in compliance  with all  applicable
laws and ordinances,  and all covenants,  conditions and restrictions applicable
to the Building. If any prohibited sign, advertisement or notice is nevertheless
exhibited  by Tenant,  Landlord  shall  have the right to remove  the same,  and
Tenant  shall pay any and all  expenses  incurred by  Landlord in such  removal,

<PAGE>

together   with   interest   thereon  at  the   Interest   Rate,   upon  demand.
Notwithstanding  anything contained herein to the contrary,  Landlord shall have
the right to  prohibit  any sign  posted at the  Building  by Tenant  which,  in
Landlord's  opinion,  tends to impair  the  reputation  of the  Building  or its
desirability as a first class office building.

         14.2 SPECIAL SIGN RIGHTS. Section 14.1 to the contrary notwithstanding,
but in all instances subject (A) to Landlord's prior approval (which will not be
unreasonably  withheld,  conditioned  or delayed) as to the size,  type,  color,
design,  location and method of attachment  thereof,  and (B) to compliance with
all  applicable  laws  and  ordinances,   and  all  covenants,   conditions  and
restrictions  applicable to the Building: (1) Tenant is hereby granted exclusive
signage rights on the Building, which will permit the installation by Tenant, at
Tenant's sole expense, of a single,  prominent,  building  identification  sign,
identifying Tenant exclusively,  in such size, type, color, design, and location
as Tenant may elect  subject to  Landlord's  approval,  and (2) Tenant is hereby
granted the  non-exclusive  right,  at Tenant's sole expense,  to install a sign
panel on the Building's  current  monument sign which may utilize  Tenant's name
and/or corporate logo, but which shall otherwise  conform to the general design,
size, type, and color of such monument sign as it currently exists,  and in such
location thereon as Landlord may designate.  Tenant agrees, within five (5) days
after the end of the Term of this Lease,  to remove any and all signs  installed
by it  pursuant to this Lease,  to repair any damage to the  Building  caused by
such  removal  and to  restore  any  portion  of the  Building  affected  by the
installation of such signs to its pre-existing condition.

                                   ARTICLE 15
                         TENANT'S EQUIPMENT AND PROPERTY

         15.1  MOVING  TENANT'S  PROPERTY.  Any and all  damage or injury to the
Premises or the Building  caused by moving the property of Tenant into or out of
the  Premises,  or due to the same being on the  Premises,  shall be repaired by
Landlord,  and Tenant shall  reimburse  Landlord  for the actual and  reasonable
expenses incurred in connection with such repairs.  Tenant shall promptly remove
from the Common  Area any of Tenant's  furniture,  equipment  or other  property
there deposited.

         15.2  INSTALLING  AND  OPERATING  TENANT'S  EQUIPMENT.   Without  first
obtaining  the  written  consent  of  Landlord,  which  shall be  subject to the
standards  hereinafter  set forth,  Tenant  shall not  install or operate in the
Premises (i) any electrically operated equipment or other machinery,  other than
(A) the items of equipment specifically  identified in EXHIBIT F attached hereto
and made a part hereof,  and (B) standard  equipment  for the Permitted Use that
does not require wiring, cooling or other service in excess of existing Building
standards,  (ii) any  equipment  of any kind or  nature  whatsoever  which  will
require any changes, replacements or additions to, or changes in the use of, any
water, heating,  plumbing, air conditioning or electrical system of the Premises
or the Building,  or (iii) any  equipment  which causes the floor load to exceed
the load limits set by Landlord  for the  Building.  Landlord's  consent to such
installation  or operation  will not be  unreasonably  withheld,  provided that,
without limitation, it will not be unreasonable for Landlord to deny its consent
to any such  installation,  operation or equipment (A) if the same would require
any structural modifications to the Building,  including any floor penetrations,
extra  load-bearing  enhancements or similar  structural  modifications,  (B) if
Tenant does not agree to remove all items and  equipment so  installed,  and any
and  all  Building  modifications  required  to  be  made  to  accommodate  such
installation  (and to restore the all affected portions of the Building to their
pre-existing  condition) promptly upon expiration or earlier termination of this
Lease, at Tenant's sole expense,  (C) unless Tenant agrees to pay the additional
costs for any excess consumption of utilities and any additional power,  wiring,
cooling  or  other  service  (as  determined  in the  reasonable  discretion  of
Landlord)  that may result  from such  equipment,  (D) which  involve the use of
Hazardous  Materials  except as specifically  permitted under Article 26 of this
Lease,  (E)  which  cause  noise or  vibration  that may be  transmitted  to the
structure of the Building or to any space therein so as to be  objectionable  to
Landlord  or any  other  Building  tenant,  unless  the  same is  installed  and
maintained by Tenant, at its expense, on vibration  eliminators or other devices
sufficient  to eliminate  such noise and  vibration,  and (F) if Tenant does not
agree to indemnities,  screening  requirements,  procedures and other provisions
substantially  similar  to those set forth in  Section  29.3 of this  Lease with
respect to any such installations, operations and/or equipment. Any equipment or
systems  installed by Tenant which involve  electrical  consumption in excess of
that applicable to general office use may be

<PAGE>

submetered  by Landlord  at Tenant's  expense,  and the  electrical  consumption
associated  therewith  charged back directly to Tenant (and to be paid by Tenant
on an ongoing basis within thirty (30) days after  Landlord's  written  demand).
Tenant shall be responsible for complying with all applicable laws,  ordinances,
regulations  and other legal  requirements  in connection  with any equipment or
systems  installed by Tenant  pursuant to this  section.  Tenant shall have non-
exclusive  access to the Building  risers and cableways to install,  at Tenant's
sole cost and expense,  voice, data and utility service lines in connection with
the Permitted Use, provided Tenant shall not make  disproportionate  use of such
risers and cableways (so as to ensure  reasonable  availability to other tenants
and occupants of the Building).  The Building is currently  served by electrical
service  available for Tenant use in the amount of 5.924 watts per usable square
foot of building area.

                                   ARTICLE 16
                                 RIGHT OF ENTRY

         Tenant shall permit Landlord or its Agents,  at any time and, except in
cases of  Emergency  (in which case prior  notice  shall not be  required)  upon
reasonable prior notice as hereinafter set forth, to enter the Premises, without
charge  therefor to Landlord  and without  diminution  of Rent,  (i) to examine,
inspect and protect the Premises and the Building, (ii) to make such alterations
and repairs or perform such  maintenance  which in the sole judgment of Landlord
may be deemed  necessary or desirable,  (iii) to exhibit the same to prospective
purchasers of the Building or to present or future Mortgagees or (iv) to exhibit
the same to prospective tenants during the last eighteen (18) months of the Term
and to erect on the  Premises  a  suitable  sign  indicating  the  Premises  are
available.  Landlord will use  reasonable  and diligent  efforts to minimize any
unreasonable  disruption  to  Tenant's  business  operations  during any entries
permitted under this Article 16, including coordinating such entries with Tenant
where reasonably practicable.  For purposes of this Article 16, verbal notice to
Tenant of an entry given at least one (1) Business Day prior to a  non-emergency
entry shall be deemed reasonable prior notice.

                                   ARTICLE 17
                                    INSURANCE

         17.1 INSURANCE RATING. Tenant shall not conduct or permit any activity,
or place any equipment or material,  in or about the  Premises,  the Building or
the Common Area which will  increase the rate of fire or other  insurance on the
Building or insurance  benefitting any other tenant of the Building;  and if any
increase in the rate of insurance is stated by any  insurance  company or by the
applicable  insurance  rating  bureau to be due to any  activity,  equipment  or
material of Tenant in or about the  Premises,  the  Building or the Common Area,
such  statement  shall be conclusive  evidence that the increase in such rate is
due to the same and,  as a result  thereof,  Tenant  shall pay such  increase to
Landlord upon demand.

         17.2 LIABILITY  INSURANCE.  Tenant shall, at its sole cost and expense,
procure and maintain  throughout the Term a commercial  general liability policy
insuring against claims,  demands or actions for bodily injury,  death, personal
injury, and loss or damage to property arising out of or in connection with: (i)
the Premises; (ii) the condition of the Premises;  (iii) Tenant's operations in,
maintenance and use of the Premises, Building and Common Area, and (iv) Tenant's
liability  assumed  under this Lease.  Such  insurance  shall have such combined
single limit as  reasonably  required by Landlord  from time to time,  but in no
event less than  [***] per  occurrence,  on an  occurrence  basis,  and shall be
primary over any insurance carried by Landlord.  Endorsements  shall be obtained
for cross-liability and contractual liability.

         17.3 INSURANCE FOR PERSONAL  PROPERTY.  Tenant shall,  at its sole cost
and  expense,  procure and  maintain  throughout  the Term a property  insurance
policy  (written  on an "All Risk"  basis)  insuring  all of  Tenant's  personal
property,  including  but  not  limited  to  equipment,   furniture,   fixtures,
furnishings and leasehold  improvements  which are the responsibility of Tenant,
for not less than the full  replacement  cost of said property.  All proceeds of
such  insurance  shall  be used to  repair  or  replace  Tenant's  property.  In
addition,  Tenant  shall,  at its sole cost and  expense,  procure and  maintain
business  interruption  insurance  in an amount  not less than the Base Rent due
hereunder for the first Lease Year.

         17.4 REQUIREMENTS OF INSURANCE COVERAGE. All such insurance required to
be carried by Tenant  herein shall be with an insurance  company  licensed to do
business in the  Commonwealth  of Virginia and rated not lower than A-XII in the

<PAGE>

A.M. Best Rating Guide.  Such  insurance (i) shall contain an  endorsement  that
such  policy  shall  remain in full  force and effect  notwithstanding  that the
insured has released its right of action against any party before the occurrence
of a loss; (ii) shall name Landlord and, at Landlord's request, any Mortgagee or
ground lessor, as additional  insured parties;  and (iii) shall provide that the
policy shall not be canceled, failed to be renewed or materially amended without
at least thirty (30) days' prior  written  notice to landlord and, at Landlord's
request, any Mortgagee. On or before the Commencement Date and, thereafter,  not
less than thirty (30) days before the expiration  date of the insurance  policy,
an original of the policy  (including  any renewal or  replacement  policy) or a
certified copy thereof,  together with evidence  satisfactory to Landlord of the
payment of all premiums for such policy,  shall be delivered to Landlord and, at
Landlord's request, to any Mortgagee.

         17.5 WAIVER OF SUBROGATION.  Each party hereby releases the other party
hereto  from  liability  for any loss or damage to any  building,  structure  or
tangible  personal  property,  or any resulting loss of income,  or losses under
worker's compensation laws and benefits,  notwithstanding that such loss, damage
or liability  may arise out of the  negligent or  intentionally  tortious act or
omission of the other party or its Agents,  if such loss or damage is covered by
insurance benefitting the party suffering such loss or damage or was required to
be covered by  insurance  pursuant to this Lease.  Each party  hereto  shall use
reasonable  efforts to have a waiver of subrogation  clause (providing that such
waiver of right of  recovery  against  the other  party  shall  not  impair  the
effectiveness  of such policy or the insured's  ability to recover)  included in
its said policies, and shall promptly notify the other in writing if such clause
cannot be included in any such  policy;  if such  waiver of  subrogation  clause
shall not be available,  then the foregoing waiver of right of recovery shall be
void.

         17.6  SECURITY.  In the event that  Landlord  engages the services of a
professional  security  system  for the  Building,  it is  understood  that such
engagement shall in no way increase Landlord's  liability for occurrences and/or
consequences  which such a system is designed to detect or avert and that Tenant
shall look  solely to its  insurer  as set out above for  claims for  damages or
injury to any person or property.  In no event shall this Section 17.6,  nor any
other  provision  of this Lease,  be  construed to create any express or implied
obligation  on the part of Landlord to secure the  Premises,  Building or Common
Areas,  or otherwise  provide  security  services for the Premises,  Building or
Common Areas.

         17.7 LANDLORD'S INSURANCE. At all times during the Lease Term, Landlord
will  maintain the following  insurance (a) all-risk fire and extended  coverage
casualty  insurance  covering damage to the Building  (excluding  earthquake and
flood insurance  unless  available at commercially  feasible rates) in an amount
not less than 100% of the  replacement  cost  thereof,  (b)  commercial  general
public liability insurance covering bodily injury,  death and/or property damage
in an amount  not less than [***]  combined  single  limit,  (c) loss of "rental
value"  insurance  in an amount  equal to not less 100% of the  projected  gross
income from the  Building  for a period of twelve (12)  months,  and (d) workers
compensation and employer's  liability insurance to the extent required by state
law or Landlord's  mortgage.  Landlord  shall also have the right to obtain such
other types and amounts of insurance  coverage on the  Building  and  Landlord's
liability in connection with the Building as Landlord determines is customary or
advisable  for  comparable   office   buildings  in  the  Reston  area.   Tenant
acknowledges  and agrees that all  premiums for  insurance  obtained by Landlord
pursuant to this Section 17.7 shall be included within "Operating Expenses",  as
such term is defined in Section  7.2,  above.  Landlord  shall have the right to
provide insurance  coverage which it is obligated to carry pursuant to the terms
hereof in a  blanket  policy  or  through  the use of  primary  and/or  umbrella
coverages.

                                   ARTICLE 18
                         LANDLORD SERVICES AND UTILITIES

         18.1 ORDINARY  SERVICES TO THE PREMISES.  Landlord shall furnish to the
Premises  throughout  the  Term,  at a  standard  consistent  with that of other
comparable  first class  office  projects in the  vicinity of the  Building  (i)
heating and air conditioning appropriate for the Permitted Use between 7:00 a.m.
and 7:00 p.m.,  Monday  through  Friday,  and between 9:00 a.m. and 1:00 p.m. on
Saturday,  except for the  following  holidays (on the days  recognized as legal
holidays  in  respect  thereof):   Christmas,  New  Year's  Day,  Memorial  Day,
Independence  Day,  Labor  Day  and  Thanksgiving,   (ii)  reasonable   in-suite
janitorial  service on Monday through  Friday of each week  throughout the Term,
excluding  the  aforementioned  holidays,  (iii)  regular trash removal from the
Premises,  (iv) hot and cold  water  from  points of supply,  (v)  restrooms  as
required by applicable code, (vi) elevator service, provided that Landlord shall

<PAGE>

have the right to remove  such  elevators  from  service  as may be for  moving,
freight or for servicing or  maintaining  the  elevators or the Building,  or on
non-business  days,  as long as one (1)  elevator  remains in  operation  at all
times,  (vii)  non-exclusive  access to the Building's  Common Area loading dock
upon reasonable  prior notice to Landlord (not less than one (1) business day in
advance), and exclusive access to that certain garage area that opens into Space
A and which is usable for certain types of loading or unloading (which,  subject
to  applicable  legal  requirements,  will be available  for Tenant's use at all
times),  and (vii)  electricity.  The cost of all services  provided by Landlord
hereunder  shall,  except as limited by Section  7.2 and 7.3 of this  Lease,  be
included within Operating  Expenses.  Landlord agrees to furnish landscaping and
grounds  maintenance  and snow  clearing  for the  areas  used in  common by the
tenants of the Building.  The foregoing  services shall be furnished by Landlord
and reimbursed by Tenant as part of Operating Expenses;  provided,  however that
Landlord  shall  be  under  no   responsibility  or  liability  for  failure  or
interruption in such services caused by breakage,  accident, strikes, repairs or
for any other cause or causes  beyond the control of Landlord,  nor in any event
for any indirect or consequential  damages; and, subject to Section 18.5, below,
failure or omission on the part of Landlord to furnish such service shall not be
construed as an eviction of Tenant,  nor work an  abatement of Rent,  nor render
Landlord liable in damages, nor release Tenant from prompt fulfillment of any of
the covenants under this Lease.

         18.2  AFTER-HOURS  SERVICES  TO THE  PREMISES.  If Tenant  requires  or
requests that the services to be furnished by Landlord (except Building standard
electricity  and  elevator  service for at least one (1)  elevator)  be provided
during  periods in  addition  to the period is set forth in Section  18.1,  then
Tenant shall obtain Landlord's  consent thereto and, if such consent is granted,
shall  pay upon  demand  Landlord's  additional  expenses  resulting  therefrom.
Landlord  may,  from time to time  during  the Term,  set a per hour  charge for
after-hours  service  which shall  include the cost of utility,  service,  labor
costs, administrative costs and a cost for depreciation of the equipment used to
provide such after-hours  service. The current rate for after hours HVAC service
is $35.00 per hour per zone.

         18.3 OTHER PROVISIONS  REGARDING  UTILITIES.  All telephone service and
utility service other than those described in Section 18.1, above, to the extent
furnished to the Premises,  shall be paid for by Tenant.  In any  multi-tenanted
Building,  to the  extent  Tenant's  special  equipment  (such as  supplementary
air-conditioning   units,  security  equipment,  or  other  equipment  involving
extraordinary  electrical  requirements)  or after hours  usage of the  Premises
involves, in Landlord's reasonable judgement,  excessive electrical  consumption
relative to other tenants of the Building, Landlord reserves the right to charge
Tenant  directly  for  the  incremental   cost  of  such  excessive   electrical
consumption, either through the use of engineering surveys or submeters or other
electrical  measurement  devices  (provided  that  Tenant  shall pay the cost of
installation of any such submeters or other  electrical  measurement  devices is
installed in order to measure Tenant's excessive electrical consumption).

         18.4  ACCESS AND  SECURITY.  Tenant  hereby  agrees to the  exercise by
Landlord and its agents and  employees,  within their sole  discretion,  of such
security  measures as Landlord deems  necessary for the Building,  provided that
such security  measures shall not interfere  with Tenant's  access to and use of
the  Premises  24 hours per day, 7 days per week,  during the Term of this Lease
(it being agreed that the operation of the current  security system which limits
entry to the Building  during  non-business  hours to persons in possession of a
security  key card or with  authorization  by  Tenant or  Landlord  to enter the
Building shall not constitute  "interference" within the meaning of this Section
18.4).  Landlord  shall provide Tenant with one hundred (100) security key cards
without  charge.  Additional  security  key cards may be obtained by Tenant from
Landlord at  landlord's  actual  cost  (without  mark-up).  Tenant may install a
security system within the Premises,  provided such system and its  installation
(i) shall be subject to Landlord's  prior written  approval,  which shall not be
unreasonably  withheld  (provided it shall not be  unreasonable  for Landlord to
deny consent to any system which is not compatible  with the building's  overall
security  and fire safety and life safety  systems,  or which is not  reasonably
usable by any successor  tenants in the  Premises),  (ii) shall be in accordance
with all  applicable  legal  requirements,  (iii) shall be performed at Tenant's
sole expense, and shall otherwise be installed in accordance with the provisions
governing Alterations under this Lease or the Work Agreement, and (iv) shall not
restrict  emergency or other  permitted  access by landlord to the Premises (and
Landlord shall be provided with at least three (3) electronic and/or master keys
which  permit  Landlord  to gain access to the  Premises at all times).  Nothing
contained in this Section 18.4 shall be construed or deemed to obligate Landlord
to  provide  any  particular  form or amount of  security  with  respect  to the
Premises  or the  Building  or on behalf of Tenant or any other  occupant  of or
visitor to the  Premises  or the  Building,  provided  that  Landlord  agrees to
maintain an electronic card-key access system to the Building.

<PAGE>

         18.5  INTERRUPTIONS  IN ESSENTIAL  SERVICES.  Section 18.1,  above, and
Section 19.1, below, to the contrary notwithstanding: (i) in the event Tenant is
deprived of  electricity,  heat,  air-conditioning,  water  and/or the use of at
least one (1)  elevator to the Premises  (any of the  foregoing  services  being
hereinafter referred to as "Essential  Services"),  Tenant shall promptly notify
Landlord  thereof and Landlord  shall use  reasonable  and  diligent  efforts to
restore (or facilitate the  restoration)  of such  Essential  Services  promptly
thereafter,  and (ii) if (A) the deprivation of such Essential Services occurred
either  solely as a result of the  willful or  negligent  acts or  omissions  of
Landlord or its  Agents,  or, if not caused  solely by the willful or  negligent
acts  or  omissions  of  Landlord  or its  Agents,  is due to a cause  which  is
nevertheless  covered  under any policy of rental  interruption  insurance  then
maintained by Landlord  with respect to the  Building,  (B) the Premises (or any
part  thereof)  is  rendered  untenantable  as a  result  thereof,  and (C) such
interruption of Essential Services  continues for five (5) consecutive  business
days after Tenant first notified Landlord of such interruption,  then commencing
on the sixth (6th) consecutive business day after the date Tenant first notified
Landlord  of such  interruption,  Tenant  shall be  entitled  to abate  its Rent
obligations  hereunder  solely as to the  portion of the  Premises  which is not
usable until such time as such Essential Service(s) is(are) restored. Other than
(i) the foregoing  abatement,  (ii) any abatement of Rent  contemplated  after a
fire or other  casualty  pursuant to Section  21.1,  below,  (iii) any sums owed
pursuant to an  indemnification  claim under  Section 19.2 of this Lease arising
out of any death or injury to persons in relation thereto, and (iv) any monetary
liability  arising  out of the  exercise  by  Tenant  of its  right  to  perform
Landlord's  obligations  after a default by  Landlord  pursuant to Article 22 of
this Lease,  Landlord shall have no monetary  liability to Tenant as a result of
any   interruption   in  services  or  Essential   Services  under  this  Lease.
Notwithstanding anything set forth in this Lease to the contrary, Landlord shall
have no liability to Tenant or any party  claiming by or through Tenant for lost
profits, business losses or other consequential damages.

                                   ARTICLE 19
                              LIABILITY OF LANDLORD

         19.1 NO  LIABILITY.  Except where due to Landlord or its Agents'  gross
negligence or willful misconduct, Landlord and its Agents shall not be liable to
Tenant or its Agents  for,  and Tenant,  for itself and its Agents,  does hereby
release Landlord and its Agents from liability for, any damage,  compensation or
claim arising from (i) the necessity of repairing any portion of the Premises or
the Building or the Common Area or any structural defects thereto,  (ii) subject
to Section  18.5,  above,  any  interruption  in the use of the  Premises or the
Common Area for any reason  including any  interruption or suspension of utility
service,  (iii) fire or other casualty or personal or property injury, damage or
loss  resulting  from the use or operation  (by Landlord,  Tenant,  or any other
person  whomsoever) of the Premises or the Building or the Common Area, (iv) the
termination  of this Lease,  (v) any robbery,  assault,  theft or other criminal
act, (vi) any leakage in the Premises or the Building from water,  rain, snow or
any other causes  whatsoever,  and/or (vii) any and all other causes whatsoever.
Except as expressly  provided in Section 18.5,  above, no such occurrence  shall
give rise to diminution or abatement of Rent. Notwithstanding the foregoing, any
goods,  automobiles,  property or personal effects stored or placed by Tenant or
its Agents in or about the Premises, the Building or the Common Area shall be at
the sole risk of Tenant (and shall be insured by Tenant pursuant to the terms of
this Lease);  and Tenant hereby  expressly  waives its right to recover  against
Landlord and its Agents  therefor.  Tenant hereby waives any claim it might have
against Landlord or its Agents for any consequential  damages or business losses
sustained by Tenant for any reason,  or arising out of the loss or damage to any
person or property of Tenant,  or any interruption in the use of the Premises or
the Common Area, for any reason.  Tenant  acknowledges  its obligation to insure
against such losses and damages.  The liability of Landlord  hereunder  shall be
further limited by Section 28.14, below.

         19.2  INDEMNITY.  Tenant  shall  indemnify,  defend,  protect  and hold
Landlord  and its Agents  harmless  from and against any and all damage,  claim,
liability, cost or expense (including, without limitation, reasonable attorneys'
or other  professionals'  fees) of every  kind and  nature  (including,  without
limitation,  those arising from any injury or damage to any person,  property or
business)  incurred by or claimed  against  Landlord or its Agents,  directly or
indirectly,  as a result of, arising from or in connection  with (i) Tenant's or
its Agents' use and occupancy of the  Premises,  the Building or the Common Area
(except to the extent caused by the negligence or willful misconduct of Landlord
and not covered under any insurance  which Tenant is required to carry  pursuant
to this Lease),  (ii) Tenant's  breach of any provision of this Lease;  or (iii)
any negligence act or omission, or willful act, of Tenant or its Agents.

<PAGE>

                                   ARTICLE 20
                              RULES AND REGULATIONS

         Tenant and its Agents shall at all times abide by and observe the Rules
and Regulations and any commercially  reasonable  amendments thereto that may be
promulgated  from time to time by Landlord for the operation and  maintenance of
the Building and the Common Area and the Rules and  Regulations  shall be deemed
to be covenants of the Lease to be performed and/or observed by Tenant.  Nothing
contained in this Lease shall be  construed to impose upon  Landlord any duty or
obligation  to enforce  the Rules and  Regulations,  or the terms or  provisions
contained in any other lease, against any other tenant of the Building, PROVIDED
Landlord  shall not enforce the Rules and  Regulations,  or promulgate new Rules
and Regulations,  in a discriminatory manner against Tenant.  Landlord shall not
be liable to Tenant for any violation by any party of the Rules and  Regulations
or the terms of any other Building lease. If there is any inconsistency  between
this Lease and the Rules and  Regulations,  this Lease  shall  govern.  Landlord
reserves  the right to amend and  modify the Rules and  Regulations  as it deems
necessary.

                                   ARTICLE 21
                              DAMAGE; CONDEMNATION

         21.1 DAMAGE TO THE PREMISES.  If the Premises  shall be damaged by fire
or other cause,  Landlord shall diligently and as soon as practicable after such
damage  occurs  repair such damage using the  proceeds of  insurance  carried by
Landlord  pursuant to this Lease  (provided that, if the fire or casualty is due
to the fault or  negligence  of  Tenant or its  Agents,  Landlord's  repair  and
restoration  obligation  shall be limited to the  amount of  insurance  actually
received  by  Landlord  in  respect  of  such  casualty).   Notwithstanding  the
foregoing,  if the Premises or the Building is damaged by fire or other cause to
such an  extent  that,  in  Landlord's  sole  judgment,  the  damage  cannot  be
substantially  repaired  within  two  hundred  (200) days after the date of such
damage,  or if the Premises are damaged to an extent such that repair would cost
in excess of $200,000  during the last two (2),  then either  Landlord or Tenant
may,  within  thirty (30) days after the date  Landlord  notifies  Tenant of its
estimate of the timing or cost of such repair, terminate this Lease by notice to
the other.  Landlord shall give Tenant written notice of Landlord's  estimate of
the time required for the substantial  repair of such damage (or, if in the last
two Lease Years,  of the estimated cost of such repairs) within thirty (30) days
after the date of such  damage.  If either  Landlord or Tenant  terminates  this
Lease pursuant to this Article 21, the Rent shall be apportioned and paid to the
date of such termination.  If neither Landlord nor Tenant so elects to terminate
this Lease but the damage  required to be  repaired by Landlord is not  repaired
within two  hundred  (200)  days from the date of such  damage,  or such  longer
period as was set forth in  Landlord's  estimate  (such period to be extended by
the  period of any delay  resulting  from Force  Majeure,  as defined in Section
28.16,  and by any period of delay  caused by the acts or omissions of Tenant or
Tenant's  Agents),  Tenant may terminate this Lease at any time thereafter until
such  reconstruction  is completed upon thirty (30) days prior written notice to
Landlord,  PROVIDED that if such reconstruction is substantially  underway as of
the  date  of  Tenant's   notice  of  termination  to  Landlord,   and  Landlord
substantially  completes such  reconstruction  within the thirty (30) day period
after its  receipt of  Tenant's  notice of  termination,  then in such event the
notice of  termination  from Tenant  shall be deemed  rescinded,  and this Lease
shall  continue  in full force and  effect.  During the  period  that  Tenant is
deprived  of the use of the  damaged  portion  of the  Premises,  Base  Rent and
Additional  Rent shall be reduced by the ratio that the rentable  square footage
of the Premises damaged bears to the

<PAGE>

total   rentable   square   footage  of  the   Premises   before  such   damage.
Notwithstanding   anything  herein  to  the  contrary,   Landlord's  repair  and
restoration  obligation shall be limited to the Tenant  Improvements  originally
constructed  by Landlord  hereunder,  and shall not apply to (and Landlord shall
not be required to rebuild,  replace or repair) any  Alterations,  any  personal
property and/or any specialized equipment of Tenant.

         21.2  CONDEMNATION.  If the whole or a Substantial Part of the Premises
or  the  Building   shall  be  taken  or  condemned  by  any   governmental   or
quasi-governmental  authority  for any  public or  quasi-public  use or  purpose
(including,  without limitation,  sale under threat of such a taking),  then the
Term  shall  cease  and  terminate  as of the  date  when  title  vests  in such
governmental or quasi-governmental  authority, and Rent shall be prorated to the
date when title vests in such governmental or quasi-governmental  authority.  If
less  than a  Substantial  Part of the  Premises  is taken or  condemned  by any
governmental or quasi-governmental  authority for any public or quasi-public use
or purpose (including,  without limitation, sale under threat of such a taking),
Base Rent shall be reduced  on a per  square  foot basis for the  portion of the
Premises so taken, and Tenant's Proportionate Share shall be adjusted to reflect
the actual ratio of the remaining rentable area of the Premises to the remaining
rentable area of the Building  after such taking,  effective as of the date when
title vests in such governmental or quasi-governmental authority, and this Lease
shall  otherwise  continue  in  full  force  and  effect.   Notwithstanding  the
foregoing,  if more than 25% of the Premises is taken,  or areas of the Premises
are taken that can not  reasonably be rebuilt in other areas of the Premises and
are vital to Tenant's  business  operations (such as Tenant's network  operating
center or interconnection  spaces), Tenant may terminate this Lease upon written
notice to Landlord  delivered  within  thirty (30) days after the date Tenant is
first notified of such condemnation. Tenant shall have no claim against Landlord
(or otherwise) as a result of any taking described in this Section 21.2.  Tenant
may, to the extent  allowed by law,  claim an award for moving  expenses and for
the taking of any of Tenant's  property  which does not, under the terms of this
Lease,  become the  property of Landlord at the  termination  hereof (but in all
events specifically  excluding Tenant's leasehold interest in the Premises),  as
long as such claim is separate and distinct  from any claim of Landlord and does
not diminish Landlord's award. Any award for the termination of this Lease shall
be the sole property of Landlord.

                                   ARTICLE 22
                                     DEFAULT

         22.1 EVENTS OF  DEFAULT.  Each of the  following  shall  constitute  an
"Event of  Default":  (i) Tenant fails to pay Rent within  two (2) business days
after notice from Landlord; provided that no such notice shall be required if at
least five (5) such  notices  shall have been given  during the same Lease Year;
(ii) Tenant  fails to observe or perform any other term,  condition  or covenant
herein  binding upon or  obligating  Tenant  within ten (10) days after  written
notice from  Landlord,  or such longer period (not to exceed an  additional  one
hundred eighty (180) days) as may be reasonably  necessary to cure such failure,
provided  Tenant  commences  such cure promptly  after its receipt of Landlord's
written  notice and  diligently  continues  such cure  thereafter to completion,
(iii)  Tenant  abandons  the  Premises;  (iv) Tenant or any  Guarantor  makes or
consents to a general  assignment  for the benefit of  creditors or a common law
composition of creditors,  or a receiver of the Premises or all or substantially
all of Tenant's or Guarantor's  assets is appointed,  or (v) Tenant or Guarantor
files a voluntary  petition in any  bankruptcy or insolvency  proceeding,  or an
involuntary petition in any bankruptcy or insolvency proceeding is filed against
Tenant or Guarantor and is not  discharged by Tenant or Guarantor  within ninety
(90) days.

         22.2 LANDLORD'S  REMEDIES.  Upon the occurrence of an Event of Default,
Landlord, at its option,  without further notice or demand to Tenant, may to the
fullest  extent  permitted  by law,  and in  addition  to all other  rights  and
remedies provided in this Lease, at law or in equity:

         (i)  Terminate  this  Lease and  Tenant's  right of  possession  of the
Premises,  and  recover all  damages to which  Landlord  is entitled  under law,
specifically  including,  but without limitation,  all of Landlord's expenses of
reletting  (including,  without  limitation,  rental concessions to new tenants,
repairs,  Alterations,  reasonable  legal fees and  brokerage  commissions).  In
addition,  if Landlord  elects to  terminate  this Lease,  every  obligation  of
Landlord hereunder shall cease as of the date of such termination,  but the same
shall not affect the liability of Tenant for payment of Rent and  performance of
all other terms and  conditions of this Lease to the date of  termination,  plus
any damages  and/or other sums  recoverable by Landlord under this Article 22 or
otherwise at law or in equity.

<PAGE>

         (ii)  Terminate  Tenant's  right of possession of the Premises  without
terminating this Lease, in which event, Landlord may, but shall not be obligated
to, relet the Premises, or any part thereof, for the account of Tenant, for such
rent and term and upon such other conditions as are acceptable to Landlord.  For
purposes of such reletting, Landlord is authorized to redecorate,  repair, alter
and improve the Premises to the extent  necessary in Landlord's sole discretion.
Until Landlord relets the Premises, Tenant shall remain obligated to pay Rent to
Landlord as provided in this Lease.  If and when the Premises are relet and if a
sufficient  sum is  not  realized  from  such  reletting  after  payment  of all
Landlord's  expenses  of  reletting  (including,   without  limitation,   rental
concessions  to new tenants,  repairs,  Alterations,  reasonable  legal fees and
brokerage  commissions)  to satisfy the payment of Rent due under this Lease for
any month,  Tenant shall pay Landlord any such  deficiency  upon demand.  Tenant
agrees that  Landlord may file suit to recover any sums due Landlord  under this
Section  from time to time and that  such suit or  recovery  of any  amount  due
Landlord  shall not be any  defense to any  subsequent  action  brought  for any
amount not previously reduced to judgment in favor of Landlord.

         (iii)  Terminate  this Lease and Tenant's  right of  possession  of the
Premises,  and recover from Tenant,  at Landlord's  sole and  exclusive  option,
either (A) the net  present  value of the Rent due from the date of  termination
until the Expiration  Date,  discounted at the lesser of the Interest Rate as of
the date of termination or seven percent (7%) per annum, LESS the amount of rent
loss Tenant  proves  Landlord  could  reasonably  avoid during such period (also
discounted  to present  value on the same basis),  or (B)  "Indemnity  Payments"
which shall mean an amount equal to the Base Rent and Additional  Rent and other
payments  provided  for in this  Lease  which  would  have  become due and owing
thereunder from time to time during the unexpired Lease Term after the effective
date of the  termination,  but for such  termination,  less  the  Base  Rent and
Additional Rent and other payments,  if any, actually  collected by Landlord and
allocable to the Premises.  If Landlord elects to pursue  Indemnity  Payments in
lieu of the amount  recoverable  under  clause (A) of this  subparagraph  (iii),
above,  Tenant shall, on demand,  make Indemnity Payments monthly,  and Landlord
may sue for all  Indemnity  Payments  at any  time  after  they  accrue,  either
monthly, or at less frequent intervals.  Tenant further agrees that Landlord may
bring  suit for  Indemnity  Payments  at or after the end of the  Lease  Term as
originally contemplated under this Lease, and Tenant agrees that, in such event,
Landlord's cause of action to recover the Indemnity  Payments shall be deemed to
have accrued on the last day of the Lease Term as originally contemplated.

         (iv)  Re-enter  and  repossess  the Premises and remove all persons and
effects  therefrom,  by summary  proceeding,  ejectment  or other legal  action.
Landlord shall have no liability by reason of any such re-entry, repossession or
removal.

         (v) Recover from Tenant, to the extent not already  recovered  pursuant
to the preceding  subparagraphs  of this Section 22.2, any other amount which is
necessary to compensate  Landlord for economic damages sustained by Landlord and
caused by Tenant's  failure to perform  Tenant's  obligations  under this Lease,
including,  without limitation,  any costs or expenses incurred by Landlord: (i)
in  retaking  possession  of  the  Premises;  (ii)  in  maintaining,  repairing,
preserving,  restoring,  replacing,  cleaning,  altering or  rehabilitating  the
Premises or a portion thereof,  including  expenses  incurred in performing such
acts in  connection  with any  reletting  to a new tenant or tenants;  (iii) for
leasing commissions incurred in connection with any reletting to a new tenant or
tenants;  and/or (iv) for any other costs  incurred by Landlord in reletting the
Premises.

         22.3 RIGHTS UPON POSSESSION.  If Landlord takes possession  pursuant to
this Article,  with or without this Lease,  Landlord  may, at its option,  enter
into the  Premises,  remove  Tenant's  Alterations,  signs,  personal  property,
equipment  and other  evidences of tenancy,  and store them at Tenant's risk and
expense or dispose of them as Landlord may see fit, and take and hold possession
of the Premises;  provided,  however, that if Landlord elects to take possession
only  without  terminating  this  Lease,  such  entry and  possession  shall not
terminate this Lease or release  Tenant or any  Guarantor,  in whole or in part,
from the obligation to pay the Rent reserved hereunder for the full Term or from
any other obligation under this Lease or any guaranty thereof.

         22.4 NO WAIVER. If Landlord shall institute  proceedings against Tenant
and a  compromise  or  settlement  thereof  shall be made,  the same  shall  not
constitute  a waiver  of any  other  covenant,  condition  or  agreement  herein
contained,  nor of any of Landlord's rights hereunder.  No waiver by Landlord of
any breach shall operate as a waiver of such  covenant,  condition or agreement,
or operate as a waiver of such covenant,  condition or agreement  itself,  or of
any  subsequent  breach  thereof.  No payment of Rent by Tenant or acceptance of
Rent by  Landlord  shall  operate as a waiver of any breach or default by Tenant
under this  Lease.  No payment by Tenant or receipt by Landlord of a lesser than
the monthly  installment of Rent herein  stipulated  shall be deemed to be other
than a payment on account of the earliest unpaid Rent, nor shall any endorsement
or statement on any check or communication  accompanying a check for the payment
of Rent be deemed an accord and satisfaction, and Landlord may accept such check
or payment without  prejudice to Landlord's right to recover the balance of such
Rent or to pursue any other  remedy  provided  in this  Lease.  No  re-entry  by
Landlord, and no acceptance by Landlord of keys from Tenant, shall be considered
an acceptance of a surrender of the Lease.

<PAGE>

         22.5 RIGHT OF LANDLORD TO CURE TENANT'S DEFAULT. If an Event of Default
shall occur, then Landlord may (but shall not be obligated to) make such payment
or do such act to cure the  Event of  Default,  and  charge  the  amount  of the
expense thereof, together with interest thereon at the Interest Rate, to Tenant.
Such payment shall be due and payable upon demand;  however,  the making of such
payment or the taking of such action by Landlord shall not be deemed to cure the
Event of Default  or to stop  Landlord  from the  pursuit of any remedy to which
Landlord  would  otherwise  be  entitled.  Any such  payment made by Landlord on
Tenant's behalf shall bear interest until paid at the Interest Rate.

         22.6 LATE PAYMENT. If Tenant fails to pay any Rent within five (5) days
after such Rent  becomes due and  payable,  Tenant  shall pay to Landlord a late
charge of five percent (5%) of the amount of such overdue  Rent,  provided  that
the foregoing late charge shall be excused on the first occasion  Tenant makes a
late  payment of Rent as long as such  payment is made  within two (2)  business
days  after  Landlord's  written  notice  to Tenant  of such  late  payment.  In
addition, any such late Rent payment shall bear interest from the date such Rent
became due and payable to the date of payment  thereof by Tenant at the Interest
Rate  (except  that,  if Tenant was charged a late  charge with  respect to such
payment of Rent as aforesaid,  interest at the Interest  Rate shall  commence to
accrue  beginning  thirty (30) days after the date such  payment was due).  Such
late  charge and  interest  shall be due and  payable  within two (2) days after
written demand from Landlord.

         22.7 CURE PERIOD FOR LANDLORD. In no event shall Landlord be considered
to be in default of any of its obligations  under this Lease unless Tenant shall
have  notified  Landlord  in  writing  of the  alleged  breach  with  reasonable
specificity,  and such breach  shall have  continued  for thirty (30) days after
Landlord's  receipt of such  written  notice from Tenant (or, in the case of any
breach which cannot with reasonable  diligence be cured within thirty (30) days,
within such  additional  period,  if any,  but not to exceed an  additional  one
hundred  eighty (180) days in the  aggregate,  as may be reasonably  required by
Landlord to cure such breach with reasonable due diligence,  as long as Landlord
continues to pursue such cure with reasonable diligence).

                                   ARTICLE 23
                                    MORTGAGES

         23.1  SUBORDINATION.  Subject to the  delivery  of an SNDA (as  defined
herein) to Tenant in  accordance  with Section  23.4,  below,  this Lease is and
shall be subject and  subordinate to all ground or underlying  leases and to any
first  Mortgage(s)  which may now or hereafter affect such lease or the Land and
to all renewals,  modifications,  consolidations,  replacements  and  extensions
thereof.  This subordination  shall be self-operative;  however, in confirmation
thereof, Tenant shall execute promptly any instrument that Landlord or any first
Mortgagee  may  request  confirming  such  subordination.   Notwithstanding  the
foregoing, before any foreclosure sale under a

<PAGE>

Mortgage, the Mortgagee shall have the right to subordinate the Mortgage to this
Lease, and, in the event of a foreclosure, this Lease may continue in full force
and  effect  and  Tenant  shall  attorn to and  recognize  as its  landlord  the
purchaser  of  Landlord's  interest  under this Lease.  Tenant  shall,  upon the
request of a Mortgagee or purchaser at  foreclosure,  execute,  acknowledge  and
deliver any instrument  consistent  with the provisions of Section 23.4,  below,
that  has for its  purpose  and  effect  the  subordination  of the  lien of any
Mortgage to this Lease or Tenant's attornment to such Purchaser.

         23.2  MORTGAGEE  PROTECTION.  Tenant  agrees to give any  Mortgagee  by
certified mail, return receipt requested, a copy of any notice of default served
upon  Landlord,  provided  that before such notice  Tenant has been  notified in
writing of the address of such Mortgagee. Tenant further agrees that if Landlord
shall have  failed to cure such  default  within the time  provided  for in this
Lease,  then Mortgagee shall have an additional thirty (30) days within which to
cure such default; provided,  however, that if such default cannot be reasonably
cured within that time,  then such Mortgagee  shall have such additional time as
may be necessary to cure such default so long as Mortgagee  has commenced and is
diligently  pursuing  the remedies  necessary  to cure such default  (including,
without limitation, the commencement of foreclosure proceedings,  if necessary),
in which event this Lease  shall not be  terminated  or Rent  abated  while such
remedies are being so diligently  pursued.  In the event of the sale of the Land
or the  Building,  by  foreclosure  or deed in lieu  thereof,  the  Mortgagee or
purchaser  at such sale  shall be  responsible  for the  return of the  Security
Deposit only to the extent that such  Mortgagee or purchaser  actually  received
the Security Deposit.

         23.3  MODIFICATION  DUE TO FINANCING.  If, in connection with obtaining
construction or permanent financing for the Premises,  the Building or the Land,
any lender (or Mortgagee) shall request  reasonable  modifications of this Lease
as a condition to such financing,  Tenant shall promptly  execute a modification
of this Lease,  provided such  modifications  do not increase the obligations of
Tenant hereunder or materially  adversely  affect the leasehold  interest hereby
created or Tenant's reasonable use and enjoyment of the Premises or detract from
Tenant's rights under this Lease in more than a DE MINIMIS  fashion.  Tenant and
any Guarantor  shall each,  prior to execution  and  throughout  the Term,  upon
request from time to time, provide such financial  information and documentation
about itself to Landlord or Mortgagee as may be requested.

         23.4  NON-DISTURBANCE  AGREEMENT.  Landlord  agrees to obtain  from any
future  Mortgagee  for  the  Building,  a  subordination,   non-disturbance  and
attornment  agreement using such Mortgagee's  standard form (such agreement,  an
"SNDA"),  providing, INTER ALIA, (i) for the subordination of this Lease to such
Mortgage,  (ii) for the  attornment of Tenant to Landlord's  successor in title,
(iii) that, as long as Tenant is not in Default  hereunder beyond any applicable
notice and cure period,  Tenant's right of possession and other leasehold rights
shall not be disturbed in the event of a foreclosure  of such Mortgage or a sale
in lieu of foreclosure,  and (iv) that if Landlord  defaults in its construction
obligations  pursuant to EXHIBIT B of this Lease, and provided the mortgagee has
been given  written  notice of such default and an  opportunity  to cure same as
otherwise  provided  herein and has failed or declined to do so, such  mortgagee
shall recognize the remedies provided to Tenant pursuant to EXHIBIT B in respect
of such default.  Landlord will cooperate with Tenant to negotiate with a future
Mortgagee the terms and  provisions of any future SNDA,  but gives no assurances
that it will be able to secure a future  lender's  agreement to deviate from its
then standard form of SNDA in any respect, and Landlord's  obligation to deliver
a future SNDA as a condition to Tenant's subordination of this lease to a future
Mortgage shall be strictly  limited to causing such Mortgagee to deliver an SNDA
on such future  Mortgagee's  standard  form.  Upon delivery of an SNDA to Tenant
providing  for the  foregoing,  and  containing  such other  provisions  as such
Mortgagee may require under its standard form, Tenant agrees to promptly execute
and deliver such SNDA to Landlord (and prior to its execution by the Mortgagee).
In addition,  Landlord shall obtain an SNDA from Landlord's present mortgagee in
the form of EXHIBIT E attached  hereto within thirty (30) days after the date of
execution  of this  Lease  (provided  Tenant  shall  execute  such SNDA prior to
execution by such mortgagee).

                                   ARTICLE 24
                             SURRENDER; HOLDING OVER

         24.1 SURRENDER OF THE PREMISES.  Tenant shall  peaceably  surrender the
Premises to  Landlord  on the  Expiration  Date or earlier  termination  of this
Lease,  in  broom-clean  condition and in as good  condition as when Tenant took
possession,  including,  without  limitation,  the  repair of any  damage to the

<PAGE>

Premises  caused by the  removal of any of Tenant's  personal  property or trade
fixtures  from the  Premises,  except  for  reasonable  wear and tear and damage
caused by fire or other casualty.  Any of Tenant's  personal property left on or
in the Premises,  the Building or the Common Area after the  Expiration  Date or
earlier  termination  of this Lease  shall be deemed to be  abandoned,  and,  at
Landlord's option, title shall pass to Landlord under this Lease.

         24.2  HOLDING  OVER.  In the event that  Tenant  shall not  immediately
surrender the Premises to Landlord on the Expiration Date or earlier termination
of this Lease,  Tenant shall be deemed to be a month to month tenant upon all of
the terms and  provisions  of this Lease,  except the monthly Base Rent shall be
[***] of the  monthly  Base Rent in effect  during  the last  month of the Term.
Notwithstanding  the  foregoing,  if Tenant shall hold over after the Expiration
Date or earlier  termination of this Lease,  and Landlord shall desire to regain
possession  of the  Premises,  then  Landlord  may  forthwith  re-enter and take
possession  of the  Premises.  If the Premises are not  surrendered  as and when
aforesaid,  and in  accordance  with  the  terms  of this  Lease,  Tenant  shall
indemnify  Landlord against all liabilities and damages sustained by Landlord by
reason of such retention of possession.

                                   ARTICLE 25
                                 QUIET ENJOYMENT

         25.1 COVENANT OF QUIET  ENJOYMENT.  Landlord  covenants  that if Tenant
shall pay Rent and perform all of the terms and  conditions  of this Lease to be
performed by Tenant,  Tenant shall during the Term  peaceably and quietly occupy
and enjoy  possession  of the  Premises  without  molestation  or  hindrance  by
Landlord  or any  party  claiming  through  or under  Landlord,  subject  to the
provisions of this Lease and any and all easements, conditions, restrictions and
other matters of record affecting the Land and/or Building.

25.2 LANDLORD'S  WARRANTY OF TITLE.  Landlord hereby  represents and warrants to
Tenant that it is the sole owner of the Building and the Land, and that there is
no ground lease on or affecting either the Building or the Land.

                                   ARTICLE 26
                TENANT'S COVENANTS REGARDING HAZARDOUS MATERIALS

         26.1 DEFINITION.  As used in this Lease, the term "Hazardous  Material"
means any flammable items, explosives, radioactive materials, hazardous or toxic
substances,  material or waste or related  materials,  including any  substances
defined as or included in the definition of "hazardous  substances,"  "hazardous
wastes," "infectious wastes," "hazardous materials" or "toxic substances" now or
subsequently  regulated under any federal,  state or local laws,  regulations or
ordinances including, without limitation, oil, petroleum-based products, paints,
solvents,  lead,  cyanide,  DDT,  printing  inks,  acids,  pesticides,   ammonia
compounds and other chemical products, asbestos, PCBs and similar compounds, and
including any different  products and materials which are subsequently  found to
have adverse effects on the environment or the health and safety of persons.

         26.2     GENERAL PROHIBITION AND PERMITTED MATERIALS.

                  26.2.1 Tenant shall not cause or permit any Hazardous Material
to be generated,  produced,  brought upon, used,  stored,  treated,  discharged,
released,  spilled or  disposed  of on,  in,  under or about the  Premises,  the
Building  or the  Land  by  Tenant  or its  Agents,  affiliates,  sublessees  or
assignees.  Tenant shall indemnify,  defend and hold Landlord  harmless from any
and all actions (including, without limitation,  remedial or enforcement actions
of any kind,  administrative  or judicial  proceedings,  and orders or judgments
arising out of or  resulting  therefrom),  costs,  claims,  damages  (including,
without limitation,  punitive damages), expenses (including, without limitation,
reasonable  attorneys',  consultants' and experts' fees, court costs and amounts
paid in settlement of any claims or actions), fines, forfeitures or other civil,
administrative or criminal penalties, injunctive or other relief (whether or not
based upon personal or bodily  injury,  property  damage,  contamination  of, or
adverse  effects  upon,  the  environment,  water tables or natural  resources),
liabilities or losses arising from a breach of this  prohibition by Tenant,  its
Agents,  affiliates,  sublessees  or  assignees.  Except as set forth in Section
29.3,  below,  in no  event  shall  Landlord  be  required  to  consent  to  the
installation  or use of any  storage  tanks in, on or under  the  Premises,  the
Building or the Land. If Landlord consents to the generation,  production,  use,
storage,  treatment or disposal of Hazardous  Materials in or about the Premises
by Tenant, its Agents, affiliates, sublessees or assignees, then, in addition to
any other requirements or conditions that Landlord may impose in connection with

<PAGE>

such  consent,  (1) Tenant  promptly  shall  deliver to  Landlord  copies of all
permits,  approvals,  filings,  and  reports  reflecting  the legal  and  proper
generation,  production,  use,  storage,  treatment or disposal of all Hazardous
Materials  generated,  used, stored,  treated or removed from the Premises,  the
Building and the Land and,  upon  Landlord's  request,  copies of all  hazardous
waste manifests relating thereto, and (2) upon expiration or earlier termination
of this Lease,  Tenant  shall cause all  Hazardous  Materials  arising out of or
related  to the use or  occupancy  of the  Premises  by  Tenant  or its  Agents,
affiliates,  sublessees  or  assignees  to be  removed  from the  Premises,  the
Building and the Land and transported for use, storage or disposal in accordance
with all applicable  laws,  regulations  and ordinances and Tenant shall provide
Landlord with evidence reasonably satisfactory to Landlord of the same.

                  26.2.2   PERMITTED MATERIALS.

                           26.2.2.1  Notwithstanding  the foregoing,  Tenant and
its permitted  assignees and subtenants shall be permitted to use, store, handle
and dispose of reasonable amounts of Hazardous Materials that are typically used
in the operation of the Permitted  Use, such as ordinary  cleaners,  printer and
duplication supplies and similar materials (the "Permitted  Materials") provided
such Permitted  Materials are used, stored,  handled and disposed of in a manner
meeting the  requirements of all  Environmental  Laws. Any such use, storage and
disposal  shall be subject to all of the terms of this  Section  (except for the
terms  prohibiting  same),  and Tenant shall be  responsible  for  obtaining any
required  permits and paying any fees and providing any testing  required by any
governmental agency with respect to the Permitted Materials.

                           26.2.2.2    Landlord    acknowledges   that  Tenant's
telecommunications  use  will  involve  the use of  certain  equipment  that may
involve the use, storage,  handling and disposal of minimal amounts of Hazardous
Materials  that are necessary for the operation of such  equipment.  Attached as
EXHIBIT F hereto is a complete  list of all  equipment to be installed by Tenant
in the  Premises  or  otherwise  pursuant  to  this  Lease  that  would  involve
compliance  with the  provisions  of this Article 26, and a  description  of the
Hazardous  Materials,  if any,  that are  required to be used,  stored , handled
and/or  disposed of in connection  therewith,  and Landlord agrees that the use,
storage,  handling and/or disposal of minimal amounts of Hazardous  Materials as
necessary  in  connection  with the  operation  of the items listed on EXHIBIT F
shall be permissible solely to the extent described therein, and subject further
to Tenant's  compliance  with all of the provisions of this Article 26 exclusive
of provisions  prohibiting  same (and such Hazardous  Materials  shall be deemed
"Permitted  Materials",  but solely to the extent  required to be used,  stored,
handled or disposed of in the operation of the equipment  listed on EXHIBIT F or
as otherwise permitted hereinbelow).  Without limitation, and in addition to the
covenants  and  restrictions  set forth in Section 29.3 (with regard to Tenant's
Generator  and Generator  Fuel Tank),  Tenant agree that it will comply with all
applicable Environmental Laws in connection with the use, storage,  handling and
disposal of all such  Permitted  Materials.  Any such use,  storage and disposal
shall be  subject  to all of the  terms of this  Section  (except  for the terms
prohibiting  same),  and Tenant shall be responsible  for obtaining any required
permits  and  paying  any  fees  and  providing  any  testing  required  by  any
governmental agency with respect to such Permitted Materials.

                           26.2.2.3   If  Landlord  in  its  reasonable  opinion
determines  that any Permitted  Materials  are being  improperly  stored,  used,
handled or disposed  of,  then Tenant  shall  immediately  take such  corrective
action  as may be  reasonably  requested  by  Landlord.  Should  Tenant  fail to
commence such  corrective  action within two (2) business days and complete such
corrective  action  within ten (10)  business days (or (i) such longer period as
may be necessary in the exercise by Tenant of reasonable and diligent efforts to
complete such  corrective  action without delay,  or (ii) such shorter period as
may be appropriate in the event of an Emergency),  Landlord shall have the right
to perform such work on Tenant's behalf and at Tenant's sole expense, and Tenant
shall promptly  reimburse  Landlord for any and all costs  associated  with said
work.

                           26.2.2.4   Landlord   acknowledges   that   Tenant
anticipates the possibility that developments in the telecommunications business
may require Tenant to upgrade,  modify or add to the equipment listed on EXHIBIT
F and which may also necessarily involve the use, storage,  handling or disposal
of minimal  amounts  Hazardous  Materials.  Tenant  agrees  that any such future
modifications  and  activities  will,  to the extent that they  involve the use,
storage,  handling or disposal of  Hazardous  Materials  in any amount,  require
Landlord's prior written approval, which may, without limitation,  be denied (A)
if such proposed use involves use, storage,  handling or disposal of more than a
minimal amount of Hazardous  Materials  which are necessary for the operation of
such future  equipment,  (B) if such proposed use does not  constitute a normal,
safe   and   necessary   equipment    requirement    typically   applicable   to

<PAGE>

telecommunications  uses of the type  operated  by  companies  similar to Tenant
within  an  office  building  environment,  (C) if such  proposed  use is not in
compliance with all rules,  regulations,  statutes and codes of any governmental
authority  having  jurisdiction  thereover,  (D) if such  proposed use is not in
compliance  with any covenants,  conditions and  restrictions  applicable to the
Building,  (E) if Landlord  has any  significant  objection  to the  location or
method of installation of such equipment, the particular specifications thereof,
(F) if the nature of the risk associated with the use of Hazardous  Materials in
connection with such equipment (including risk of bodily injury, death, property
damage, spill or environmental  incident) is greater than, and not substantially
equivalent  to, that permitted  under Section  26.2.2.3 with regard to the items
initially  permitted  pursuant  to  EXHIBIT F, (G) if such  proposed  use is not
directly and  necessarily  related to Tenant's  telecommunications  use from the
Premises  (and any rights under this Section  26.2.2 shall be personal to Tenant
and shall not inure to the benefit of any  assignee or sublessee of Tenant other
than an assignee or sublessee  permitted  under Section 11.4 which  continues to
operate such telecommunications  business from the Premises), (H) if the rate of
any insurance  carried by Landlord,  or the cost of any services  required to be
carried with respect to the  Building is increased as a result  thereof,  (I) if
Tenant does not agree to provide Landlord with additional security  commensurate
with the  risks  associated  with the  installation  and use of such  equipment,
comparable  to  that  required   hereunder  with  respect  to  Tenant's  initial
equipment,  (J) if such use or installation  would violate any of the provisions
of Section  26.2.2.5  or  26.2.2.6,  below,  or (K) if Tenant  does not agree to
remove such equipment  upon the  expiration or earlier  termination of the Term,
and to restore the  Building to its  pre-existing  condition,  at Tenant's  sole
expense.

                           26.2.2.5   Landlord   makes   no  representations  or
promises of any kind  pertaining to the suitability of any area to be designated
on the  interior,  exterior or roof of the  Building  for the  installation  and
operation  of any future  equipment  by Tenant.  Tenant  will  obtain,  prior to
installation,   any  and  all  necessary  licenses,  approvals,  permits,  etc.,
necessary for the installation,  maintenance and use of any equipment  installed
within the scope of this Section 26.2.2. Tenant's installation,  operation, use,
removal and replacement of such equipment shall not in any way conflict with any
applicable law,  statute,  ordinance or governmental  rules or regulation now in
force or which may  hereafter  be  enacted.  Tenant  will,  at its sole cost and
expense,  promptly comply or ensure that the connection of such equipment to the
Building and  Premises,  and the use and  operation  thereof,  complies with all
laws, statutes,  ordinances,  governmental rules or regulations, or requirements
of any board of fire  insurance  underwriters  or other  similar  bodies  now or
hereafter  constituted relating to or affecting thereto.  Tenant shall indemnify
and hold Landlord  harmless from and against any and all loss,  cost  (including
reasonable  attorney's  fees  incurred by Landlord in enforcing  its rights with
respect  thereto),  damage or liability  arising out of any  violations  of said
laws,  statutes,  ordinances  rule or  regulations,  or arising  out of the use,
operation and maintenance of said equipment and the use,  handling,  storage and
/or disposal of any Permitted  Materials in connection  therewith.  Tenant's use
and operation of any such  equipment  shall be exercised:  (1) in such manner as
will not  create  any  hazardous  condition  or  interfere  with or  impair  the
operation of the heating, ventilation, air conditioning,  plumbing,  electrical,
fire protection, life safety, public utilities or other systems or facilities in
the Building;  (2) in such a manner as will not directly or indirectly interfere
with, delay, restrict or impose any expense, work or obligation upon Landlord in
the use or operation of such Building;  and (3) at Tenant's cost,  including the
cost of repairing  all damage to the Building  and any  personal  injury  and/or
property  damage  attributable  to  the  installation,  inspection,  adjustment,
maintenance,  removal or replacement  of any equipment,  apparatus or facilities
pursuant  hereto.  Such use shall be confined  solely to the ordinary  course of
Tenant's business  operations,  and shall be personal to Tenant and any party to
whom Tenant assigns this Lease pursuant to Section 11.4 hereof.

         26.3 NOTICE. In the event that Hazardous Materials are discovered upon,
in, or under the Premises,  the Building or the Land and any governmental agency
or entity  having  jurisdiction  over the  Premises,  the  Building  or the Land
requires the removal of such  Hazardous  Materials,  Tenant shall be responsible
for removing those Hazardous  Materials  arising out of or related to the use or
occupancy of the  Premises by Tenant or its Agents,  affiliates,  sublessees  or
assignees (but excluding any Hazardous  Materials existing in the Premises prior
to the date of this Lease, and also excluding any Hazardous Materials introduced
by Landlord, or its agents,  employees and contractors,  into the Premises, Land
or Building).  Notwithstanding the foregoing, Tenant shall not take any remedial
action in or about the  Premises,  the Building or the Land,  nor enter into any
settlement  agreement,  consent decree or other  compromise  with respect to any
claims  relating  to any  Hazardous  Material  in any  way  connected  with  the
Premises,  the Building or the Land without first notifying Landlord of Tenant's
intention to do so and affording  Landlord the opportunity to appear,  intervene
or otherwise  appropriately  assert and protect Landlord's interest with respect
thereto.  Tenant immediately shall notify Landlord in writing of: (i) any spill,
release,  discharge  or disposal of any  Hazardous  Material in, on or under the

<PAGE>

Premises,  the Building,  the Land or any portion thereof, (ii) any enforcement,
cleanup,   removal  or  other  governmental  or  regulatory  action  instituted,
contemplated  or  threatened  (if  Tenant has notice  thereof)  pursuant  to any
Hazardous  Materials  Laws;  (iii) any claim  made or  threatened  by any person
against  Tenant,  the  Premises,  the  Building or the Land  relating to damage,
contribution,  cost  recovery,  compensation,  loss or injury  resulting from or
claimed to result from any Hazardous Materials; and (iv) any reports made to any
environmental  agency  or  entity  arising  out  of or in  connection  with  any
Hazardous  Materials  in, on, under or about or removed from the  Premises,  the
Building or the Land, including any complaints,  notices,  warnings,  reports or
asserted  violations  in  connection  therewith.  Tenant  also  shall  supply to
Landlord as promptly as possible, and in any event within five (5) business days
after Tenant first  receives or sends the same,  copies of all claims,  reports,
complaints,  notices, warnings or asserted violations relating in any way to the
Premises, the Building, the Land or Tenant's use or occupancy thereof.

         26.4 SURVIVAL.  The respective  rights and  obligations of Landlord and
Tenant under this Article 26 shall survive the expiration or earlier termination
of this Lease without limitation.

                                   ARTICLE 27
                                  CONTINGENCIES

         27.1 LENDER  APPROVAL.  This Lease and the  obligations  of the parties
hereto are subject to the  approval of this Lease by the lender  which  financed
Landlord's  acquisition,  construction  and/or  ownership  of the  Property  and
Building.  Landlord shall forward an executed  counterpart of this Lease to such
lender for its  review and  approval  at the time of  delivery  of such Lease to
Landlord  executed  by Tenant.  Landlord  shall  notify  Tenant  within five (5)
business days after its receipt thereof, of whether such approval has been given
or  denied,  and,  if  denied,  the  parties  agree to  attempt in good faith to
negotiate  any  modifications  hereto in order to make this Lease  acceptable to
lender. In the event the parties are unable to reach such an agreement, and such
lender  continues  to deny its  approval,  this  Lease  shall be deemed  void AB
INITIO.  If Landlord has not delivered  such approval and the SNDA  described in
(and  pre-signed  by Tenant as set forth in) Section 23.4 hereof  within  thirty
(30) after execution of this Lease,  then at any time thereafter  until Landlord
delivers  such  approval  and the SNDA,  Tenant  may  terminate  this Lease upon
written notice to Landlord.

                                   ARTICLE 28
                                  MISCELLANEOUS

         28.1 NO  REPRESENTATION BY LANDLORD.  Tenant  acknowledges that neither
Landlord  or its Agents nor any  broker has made any  representation  or promise
with respect to the Premises,  the Building, the Land or the Common Area, except
as herein expressly set forth, and no rights, privileges,  easements or licenses
are acquired by Tenant except as herein expressly set forth. Except as otherwise
set forth herein or in EXHIBIT B of this Lease,  Tenant, by taking possession of
the  Premises  shall  accept the  Premises  and the Building "AS IS," subject to
correction of punch list items as noted pursuant to the Work  Agreement,  and to
latent defects within each Space identified no later than one (1) year after the
Commencement  Date for  each  Space,  and such  taking  of  possession  shall be
conclusive  evidence  that  the  Premises  and  the  Building  are in  good  and
satisfactory condition at the time of such taking of possession.

<PAGE>

         28.2 NO PARTNERSHIP. Nothing contained in this Lease shall be deemed or
construed to create a partnership  or joint  venture of or between  Landlord and
Tenant,  or to create any other  relationship  between Landlord and Tenant other
than that of landlord and tenant.

         28.3  BROKERS.  Landlord  recognizes  Broker(s)  as the sole  broker(s)
procuring this Lease and shall pay Broker(s) a commission therefor pursuant to a
separate  agreement  between  Broker(s) and  Landlord.  Landlord and Tenant each
represents and warrants to the other that it has not employed any broker,  agent
or finder other than Broker(s) relating to this Lease.  Landlord shall indemnify
and hold Tenant harmless, and Tenant shall indemnify and hold Landlord harmless,
from and against any claim for brokerage or other commission arising from or out
of any breach of the indemnitor's representation and warranty.

         28.4 ESTOPPEL  CERTIFICATE.  Tenant shall,  without charge, at any time
and from time to time,  within five (5) days after request therefor by Landlord,
Mortgagee,  any  purchaser of the Land or the  Building or any other  interested
person,  execute,  acknowledge  and deliver to such  requesting  party a written
estoppel certificate  certifying,  as of the date of such estoppel  certificate,
the  following:  (i) that this Lease is unmodified  and in full force and effect
(or if  modified,  that the Lease is in full  force and effect as  modified  and
setting forth such modifications); (ii) that the Term has commenced (and setting
forth the Commencement Date and Expiration Date); (iii) that Tenant is presently
occupying  the  Premises;  (iv) the  amounts  of Base Rent and  Additional  Rent
currently due and payable by Tenant;  (v) that any  Alterations  required by the
Lease to have  been made by  Landlord  have  been  made to the  satisfaction  of
Tenant;  (vi) that,  to the best of  Tenant's  knowledge,  there are no existing
set-offs,  charges,  liens,  claims or defenses  against the  enforcement of any
right hereunder, including without limitation, Base Rent or Additional Rent (or,
if alleged,  specifying the same in detail); (vii) that no Base Rent (except the
first  installment  thereof) has been paid more than thirty (30) days in advance
of its due date; (viii) that Tenant has no knowledge of any then uncured default
by  Landlord  of its  obligations  under  this  Lease  (or,  if Tenant  has such
knowledge,  specifying  the same in detail);  (ix) that, to the best of Tenant's
knowledge,  Tenant is not in  default  beyond  any  applicable  notice  and cure
periods;  (x) that the address to which  notices to Tenant  should be sent is as
set forth in the Lease (or, if not,  specifying the correct  address);  and (xi)
any other certifications reasonably requested by Landlord.

         28.5 WAIVER OF JURY TRIAL.  Landlord and Tenant each hereby waive trial
by jury in any  action,  proceeding  or  counterclaim  brought  by either  party
against the other with respect to any matter whatsoever arising out of or in any
way connected with this Lease, the relationship of Landlord and Tenant hereunder
or Tenant's use or occupancy of the Premises.  In the event  Landlord  commences
any  proceedings  for  nonpayment  of  Rent,  Tenant  shall  not  interpose  any
counterclaims other than compulsory counterclaims (E.G., counterclaims that will
be deemed  barred  unless  asserted  in the same  proceeding).  This  shall not,
however, be construed as a waiver of Tenant's right to assert such claims in any
separate action brought by Tenant.

         28.6 NOTICES. All notices or other communications hereunder shall be in
writing  and  shall be  deemed  duly  given if  delivered  in person or upon the
earlier of receipt, if mailed by certified or registered mail, or three (3) days
after  certified  or  registered  mailing,  return  receipt  requested,  postage
prepaid,  addressed and sent, if to Landlord to Landlord's  Address specified in
Section  1.15 or if to Tenant to Tenant's  Address  specified  in Section  1.16.
Landlord  and  Tenant  may from  time to time by  written  notice  to the  other
designate another address for receipt of future notices.

         28.7  INVALIDITY OF PARTICULAR  PROVISIONS.  If any  provisions of this
Lease or the  application  thereof to any person or  circumstances  shall to any
extent  be  invalid  or  unenforceable,  the  remainder  of this  Lease,  or the
application  of such provision to persons or  circumstances  other than those to
which it is invalid or unenforceable,  shall not be affected  thereby,  and each
provision  of this  Lease  shall be valid  and be  enforced  to the full  extent
permitted by law.

         28.8  GENDER  AND  NUMBER.  All  terms and  words  used in this  Lease,
regardless  of the number or gender in which  they are used,  shall be deemed to
include any other number or gender as the context may require.

<PAGE>

         28.9 BENEFIT AND BURDEN.  Subject to the  provisions  of Article II and
except as otherwise  expressly  provided,  the provisions of this Lease shall be
binding upon,  and shall inure to the benefit of, the parties hereto and each of
their respective  representatives,  heirs, successors and assigns.  Landlord may
freely and fully assign its interest hereunder.

         28.10 ENTIRE  AGREEMENT.  This Lease  (which  includes the Exhibits and
Rider attached hereto) contains and embodies the entire agreement of the parties
hereto,  and no  representations,  inducements or agreement,  oral or otherwise,
between the parties not contained in this Lease shall be of any force or effect.
This Lease (other than the Rules and Regulations, which may be changed from time
to time as provided herein) may not be modified,  changed or terminated in whole
or in part in any manner  other than by an  agreement  in writing duly signed by
Landlord and Tenant

         28.11    AUTHORITY.

                  (i) Tenant  hereby  represents  and warrants to Landlord  that
Tenant is a duly  formed and validly  existing  corporation,  in good  standing,
qualified to do business in the  Commonwealth of Virginia,  that the corporation
has full  power and  authority  to enter  into this  Lease and that he or she is
authorized to execute this Lease on behalf of the corporation.

                  (ii) Landlord  hereby  represents  and warrants to Tenant that
Landlord is a duly formed, validly existing limited liability company, qualified
to do business in the Commonwealth of Virginia,  that the company has full power
and  authority  to enter into this Lease,  and that he or she is  authorized  to
execute this Lease on behalf of the company.

         28.12  ATTORNEYS'  FEES.  If, as a result of any default of Landlord or
Tenant in its  performance  of any of the  provisions  of this Lease,  the other
party uses the services of an attorney in order to secure  compliance  with such
provisions  or recover  damages  therefor,  or to terminate  this Lease or evict
Tenant,  the  non-prevailing  party shall  reimburse the  prevailing  party upon
demand for any and all  reasonable  attorneys'  fees and expenses so incurred by
the prevailing party.

         28.13  INTERPRETATION.  This  Lease  is  governed  by the  laws  of the
Commonwealth of Virginia.

         28.14 NO PERSONAL  LIABILITY;  SALE.  Neither  Landlord nor its Agents,
whether  disclosed or undisclosed,  shall have any personal  liability under any
provision  of this Lease.  In the event of a judgment  in favor of Tenant  which
remains unpaid,  Tenant's right of redress,  execution and levy shall be limited
to the equity of Landlord  in the  Building  as  described  in Article 1 hereof,
including any net sales or insurance  proceeds thereof (I.E., after repayment of
all indebtedness secured thereby, and after payment of all costs of such sale or
insurance recovery).  In the event that the original Landlord hereunder,  or any
successor  owner  of the  Building,  shall  sell or  convey  the  Building,  all
liabilities  and  obligations  on the  part of the  original  Landlord,  or such
successor owner, under this Lease occurring thereafter shall terminate as of the
day of such sale, and thereupon all such  liabilities and  obligations  shall be
binding on the new owner.  Tenant  agrees to attorn to such new owner,  provided
such new  owner  recognizes  this  Lease  and  Tenant's  rights  hereunder.  Any
successor to Landlord's  interest  shall not be bound by (i) any payment of Base
Rent or Additional Rent made for more than one (1) month in advance,  or (ii) as
to any Mortgagee or any purchaser at foreclosure,  any amendment or modification
of this Lease made without the consent of such Mortgagee.

         28.15 TIME OF THE ESSENCE. Time is of the essence as to both Landlord's
and Tenant's obligations contained in this Lease.

<PAGE>

         28.16 FORCE MAJEURE. Except as hereafter provided, neither Landlord nor
Tenant shall be considered  to be in default of an  obligation  under this Lease
nor liable for loss or damage for  failure to perform an  obligation  (nor shall
the other party be released from any of its obligations  under this Lease if the
non-performing  party  is  delayed  in  performing  an  obligation),  where  the
performance  of such  obligation  by the  non-performing  party is  delayed as a
result of event of "Force  Majeure"  which shall mean any acts of God,  strikes,
lockouts, labor difficulties,  materials shortages, explosions, sabotage, riots,
civil commotions,  acts of war, results of any warfare or warlike  conditions in
this or any foreign  country,  fire or casualty,  unusually  inclement  weather,
unusual governmental delays (including any processing of the building permit for
the Tenant  Improvements which exceeds six (6) weeks after initial  submission),
legal  requirements,  energy  shortages  or other causes  beyond the  reasonable
control  of the non-  performing  party,  provided  that in no event  shall  (i)
financial  inability be  considered  an event of Force  Majeure,  and (ii) in no
event  shall  Force  Majeure  excuse  the  timely  performance  by Tenant of its
monetary obligations under this Lease.

         28.17 HEADINGS.  Captions and headings are for convenience of reference
only.

         28.18  MEMORANDUM OF LEASE.  Tenant shall,  at the request of Landlord,
execute and deliver a memorandum of lease in recordable  form.  Tenant shall not
record such a memorandum or this Lease without Landlord's  consent. In the event
Tenant  requests  recordation  of a  memorandum  of this Lease,  Tenant shall be
obligated  to pay all  costs,  fees and  taxes,  if any,  associated  with  such
recordation.

         28.19  FINANCIAL  INFORMATION.  Within ten (10) days  after  Landlord's
request,   Tenant  shall  deliver  to  Landlord  unaudited  quarterly  financial
statement  for Tenant in respect of its most recent  fiscal  quarter and (to the
extent not previously  delivered by Tenant to Landlord)  Tenant's audited annual
financial statement for its two (2) most recent fiscal years. Such quarterly and
annual  financial  statements shall include,  at a minimum,  a balance sheet, an
income statement, and a statement of change in financial position or sources and
uses of cash,  together with any accompanying  notes.  Tenant hereby agrees that
Tenant's annual financial  statements shall be completed within ninety (90) days
after Tenant's fiscal year-end and that Tenant's quarterly financial  statements
shall  be  completed   within   forty-five   (45)  days  after  Tenant's  fiscal
quarter-end. The certified public accountant preparing any such annual financial
statement shall provide an opinion that such financial statement is complete and
materially  accurate  and that the same has been  prepared  in  accordance  with
generally accepted accounting principles consistently applied.

         28.22 EFFECTIVENESS. The furnishing of the form of this Lease shall not
constitute an offer and this Lease shall become effective upon and only upon its
execution by and delivery to each party hereto.

                                   ARTICLE 29
                               SPECIAL PROVISIONS

         29.1     RENEWAL OPTION(S).

                  29.1.1  Provided  Tenant has not committed a monetary or other
Default of this Lease beyond any  applicable  cure period within the [***] prior
to its  exercise of the Renewal  Option  (defined  below) or between the date of
such exercise and the  commencement  of the Renewal Term,  Tenant shall have the
option (a "Renewal Option") to extend the Lease Term for one (1) period of [***]
months  (referred to as the "Renewal Term") provided Tenant gives written notice
to  Landlord  of its  election to exercise  such  Renewal  Option (the  "Renewal
Notice")  not less than twelve (12) months prior to the  expiration  of the last
day of the initial Lease Term or, as applicable, the initial Renewal Term.

                  29.1.2  All  terms and  conditions  of this  Lease,  including
without  limitation,  all provisions  governing the payment of Additional  Rent,
shall  remain in full force and effect  during the Renewal  Term(s),  except the
Base Rent shall be as set forth in this Section 29.1.

                  29.1.3  The Base Rent  payable  upon the  commencement  of the
Renewal  Term shall equal the  greater of (i) [***] of the Base Rent  applicable
under this Lease for the [***] Lease Year (escalated  annually thereafter at the
same annual  escalation  rate  provided for  herein),  or (ii) [***] of the then
prevailing market

<PAGE>

rental rate (including base rental rate and annual  escalation  rate) applicable
to renewal terms with respect to comparable space in comparable buildings in the
vicinity of the  Building,  (the "Fair Market Rate" or "FMR") at the time of the
commencement of the applicable Renewal Term, determined based upon then existing
renewal  market  conditions  applicable  to the leasing of  comparable  space in
comparable  buildings in the vicinity of the Building (taking into consideration
use,  location,  quality,  age and location of the  applicable  building and the
definition of net rentable  area).  Landlord and Tenant shall  negotiate in good
faith and in accordance with the procedure set forth in Section  29.1.4,  below,
to determine  the Fair Market Rate which will be  applicable  during the Renewal
Term, with the goal of concluding such negotiation or triggering a determination
of the FMR using a  three-appraiser  method (as  described  in  Section  29.1.5,
below) within not more than sixty (60) days after the date of Landlord's receipt
of the Renewal Notice.

                  29.1.4 Within ten (10) days after Landlord  receives  Tenant's
Renewal  Notice  exercising  either of the  renewal  options  referenced  above,
Landlord will provide Tenant with a written notice (the "FMR Notice") indicating
the base rental  rate and annual  escalation  rate which  Landlord in good faith
believes  represents  the then  current  FMR for the  Premises.  If Tenant is in
agreement with the base rental rate and annual escalation rate stated in the FMR
Notice,  Tenant shall so notify  Landlord within ten (10) days after its receipt
thereof,  in which case such base rental rate and annual  escalation  rate shall
constitute  the FMR for such  Renewal  Term within the  meaning of this  Section
29.1.  If Tenant  believes  in good faith that the base  rental  rate and annual
escalation rate stated by Landlord in the FMR Notice are in excess of actual FMR
for the Premises, Tenant shall so notify Landlord in writing prior to the end of
the ten (10) day period after Tenant received Landlord's FMR Notice,  stating in
its response  (hereinafter  referred to as "Tenant's  Counterproposal") the base
rental  rate and  annual  escalation  rate which  Tenant in good faith  believes
represents the then current FMR for the Premises.  If Tenant fails to respond to
the  Landlord's  FMR Notice  within  such ten (10) day period,  Tenant  shall be
deemed to have accepted the base rental rate and annual  escalation  rate stated
in Landlord's FMR Notice.  If Tenant does provide  Tenant's  Counterproposal  to
Landlord in a timely fashion,  and Landlord agrees that the base rental rate and
annual  escalation  rate stated in Tenant's  Counterproposal  represent the then
current  FMR,  Landlord  shall so notify  Tenant  within ten (10) days after its
receipt thereof,  in which case such base rental rate and annual escalation rate
shall  constitute  the FMR for such  Renewal  Term  within  the  meaning of this
Section  29.1.  If  Landlord  fails to respond to the  Tenant's  Counterproposal
within ten (10) days after Landlord's  receipt of the Tenant's  Counterproposal,
or rejects the rental rate and escalation rate stated therein,  then the parties
agree to  submit  the  issue of what  constitutes  the  appropriate  FMR for the
Premises for the Renewal Term to determination  using a "three appraiser method"
as described in Section 29.1.5, below.

                  29.1.5 If the parties submit the issue of what constitutes the
appropriate FMR for the Premises for the Renewal Term to  determination  using a
"three appraiser method", then the basic Rent and annual escalations  applicable
during the Renewal  Term shall be equal to the FMR and annual  escalation  rates
determined by a board of three (3) licensed real estate appraisers,  one of whom
shall be named by Landlord, one by Tenant, and the two so appointed shall select
the third.  Each  member of the board of  appraisers  shall be  licensed  in the
Commonwealth  of  Virginia  as a  real  estate  appraiser,  with  a  substantial
familiarity in the field of commercial office leasing in the Reston/Herndon area
of Fairfax County,  Virginia,  having no less than ten (10) years  experience in
such field, and recognized as ethical and reputable  within the field.  Landlord
and Tenant agree to make their appointments  within five (5) business days after
the  earlier to occur of (i) the  expiration  of the ten (10) day  period  after
Landlord's  receipt  of  Tenant's  Counterproposal,  or (ii) the  date  Landlord
notifies  Tenant  of its  rejection  of  Tenant's  Counterproposal.  The two (2)
appraisers  selected  by  Landlord  and  Tenant  shall  promptly  select a third
appraiser  within  ten (10) days  after  the  second  to be  appointed  has been
appointed, and each appraiser, within ten (10) days after the third appraiser is
selected,  shall submit his or her determination of the said FMR and escalations
(taking into account the provisions of Section 29.1.3 hereof).  If either of the
parties fail to select an appraiser  within the  aforesaid  time  periods,  such
party's  appraiser  shall be  designated  (in  compliance  with  the  applicable
criteria set forth above and affiliated with a different  company from the other
broker) by an agent of the  Fairfax  County  Board of Realtors in office at such
time; and if the appraisers selected by Landlord and Tenant (or on their behalf)
are unable to  reach  agreement on the identity of  the third  appraiser  within
the applicable ten (10) day period, then the third appraiser shall be designated
(in compliance with the applicable  criteria set forth above) by an agent of the
Fairfax  County  Board of Realtors in office at such time.  The FMR shall be the
average of amount  determined by the two  appraisers  whose  determinations  are
closest  in  amount  to each  other  (or if two  appraisers  reach an  identical
determination,  the determination of such two appraisers),  provided that if the

<PAGE>

two (2) most  proximate  determinations  of FMR differ by more than five percent
(5%), then the  determination  of FMR by such board of three appraisers shall be
null and void,  and Landlord  and Tenant  shall,  within five (5) business  days
thereafter,  appoint  a new  board of three  different  real  estate  appraisers
meeting the  above-stated  criteria,  who shall convene in  accordance  with the
procedures   and  time  frames  set  forth  above  in  order  to  render  a  new
determination,  as if the first  determination had never taken place.  After the
Fair Market Rent has been established,  the appraisers shall immediately  notify
the parties in writing,  and such determination  shall be conclusive and binding
upon the parties.  Landlord  and Tenant shall each pay the fee of the  appraiser
selected by it, and they shall equally share the payment of the fee of the third
appraiser.

         29.2     ROOF RIGHTS.

                  29.2.1 Subject to (i) compliance with all rules,  regulations,
statutes and codes of any governmental authority having jurisdiction  thereover,
(ii)  compliance with any covenants,  conditions and  restrictions of record (or
under applicable zoning and or community ordinances) applicable to the Building,
and  (iii)  Landlord's  prior  written  consent,  which  consent  shall  not  be
unreasonably  withheld,  conditioned or delayed,  Tenant shall have the right of
access  to and  the  non-exclusive  use of the  roof  of the  Building,  for the
installation  and  operation  of  communication  equipment,   including  without
limitation, antennas, equipment cages or shelters and associated power supplies,
generators and other  equipment  (Tenant's "Roof Use") without the obligation to
pay rent or any other fee or expense  (except for  expenses  for which Tenant is
responsible under the express terms of this Section 29.2) to Landlord in respect
of such Roof Use; provided further that such installation and the Roof Use shall
not void any roof or other  warranty  applicable  to the  Buildings and that all
such installations shall be located and screened in a manner mutually acceptable
to both Landlord and Tenant in their reasonable discretion.

                  29.2.2 If the rate of any  insurance  carried by  Landlord  is
increased  as a result of  Tenant's  Roof Use,  then Tenant will pay to Landlord
within thirty (30) days after Landlord delivers to Tenant a certified  statement
from Landlord's  insurance  carrier stating that the rate increase was caused by
Tenant's Roof Use, a sum equal to the  difference  between the original  premium
and the increased premium resulting from the Roof Use. To the best of Landlord's
knowledge,  the  installation  of a standard  satellite  dish on the roof of any
Building will not result in the rate of any insurance  carried by Landlord being
increased.

                  29.2.3 Landlord has not made any  representations  or promises
pertaining  to the  suitability  of the  Building's  rooftops  for the Roof Use.
Tenant,  for the  purpose  of this  paragraph  and its right to  rooftop  access
hereunder,  accepts  the  rooftop  in  its  "as  is"  condition.  The  foregoing
notwithstanding,  Landlord  agrees to cooperate with Tenant to designate an area
of the roof within which  Tenant's  Roof Use may be conducted  and which will be
suitable for such purpose,  provided Tenant provides appropriate  specifications
to Landlord and makes any and all  modifications  necessary to accommodate  such
Roof use (all of which shall be subject to Landlord's reasonable consent).

                  29.2.4 Tenant will obtain prior to  installation or operation,
as  appropriate,  any and all  necessary  licenses,  approvals,  permits,  etc.,
necessary for the installation,  maintenance and use of any equipment  installed
pursuant to this Section  29.2.  Tenant's Roof Use shall not in any way conflict
with any applicable law, statute,  ordinance or governmental rules or regulation
now in force or which may  hereafter be enacted.  The Tenant  will,  at its sole
cost and expense, promptly comply or ensure that Tenant's Roof Use complies with
all  laws,  statutes,   ordinances,   governmental  rules  or  regulations,   or
requirements of any board of fire insurance underwriters or other similar bodies
now or hereafter constituted.  Tenant shall indemnify and hold Landlord harmless
from and against any and all loss,  cost (including  reasonable  attorney's fees
incurred by Landlord in relation to the enforcement of this Article),  damage or
liability arising out of any violations of said laws, statutes,  ordinances rule
or regulations, or Tenant's breach of this Article 29.2.4.

<PAGE>

                  29.2.5  Tenant's  Roof  Use  shall be  exercised:  (1) in such
manner as will not create any  hazardous  condition or interfere  with or impair
the  operation  of  the  heating,   ventilation,  air  conditioning,   plumbing,
electrical,  fire protection,  life safety, public utilities or other systems or
facilities in each Building;  (2) in compliance with all applicable  laws, codes
and  regulations;  (3) in such a  manner  as will  not  directly  or  indirectly
interfere with, delay,  restrict or impose any expense,  work or obligation upon
Landlord  in the  use or  operation  of such  Building;  (4) at  Tenant's  cost,
including  the cost of repairing  all damage to the  Buildings  and any personal
injury and/or property  damage  attributable  to the  installation,  inspection,
adjustment, maintenance, removal or replacement of any equipment or apparatus on
the  roofs  approved  hereunder;  and (5) in a  manner  which  will  not void or
invalidate  any roof  warranty  then in effect with  respect to the roof of each
such Building.  Tenant's Roof Use shall be used solely in the ordinary course of
Tenant's  business  operations,  and any use of the roof outside of the ordinary
course of Tenant's business  operations (such as, but not limited to, subleasing
portions  of the roof used by Tenant for profit to third  parties,  in order for
such third parties to establish communications transmission facilities) shall be
subject to Landlord's consent, which consent shall not be unreasonably withheld,
but may be  conditioned,  INTER ALIA,  upon the payment by Tenant to Landlord of
any and all net  revenues  paid to  Tenant in  respect  thereof.  The  foregoing
notwithstanding,  Landlord  agree  that  Tenant  may  permit  third  parties  to
interconnect  directly  to  Tenant's   telecommunications  network  through  the
communications  transmission  facilities installed pursuant to Tenant's roof use
in the ordinary course of Tenant's business,  and that such interconnection will
not be subject to the immediately preceding sentence.

                  29.2.6  Any  antennas,  satellite  dishes,  cables  and  other
related  equipment  which  form a part of  Tenant's  Roof Use shall  remain  the
property  of  Tenant  except  to the  extent  paid  for  out of the  Improvement
Allowance,  and Tenant agrees, within five (5) days after the end of the Term of
this Lease, to remove all such antennas and satellite  dishes,  cables and other
related equipment,  repair any damage to the Building caused by such removal and
restore  any  portion  of  the  Building  affected  by the  installation  of the
antennas,  satellite dishes,  cables and other related equipment to its original
condition.

                  29.2.7  Landlord  shall  grant  Tenant,   its  agents  or  its
contractors,  access to the roof and other areas of the  Building at  reasonable
times to  facilitate  the  installation,  use,  maintenance  and  removal of the
antennas,  satellite dishes,  cables and other related equipment,  provided that
Tenant shall obtain  Landlord's  prior written approval of the scope of any such
work  affecting  the  roof of the  Building  and  shall be  responsible  for any
invalidation  of a roof  warranty  caused  by such  access  and  activities  (by
substituting  itself for the roof  warrantor if any such  invalidation  occurs).
Tenant  shall take  reasonable  measures  to  minimize  interference  with other
tenants  (if any) in the  Building.  Tenant,  at its  expense,  shall use a roof
contractor  approved  by  Landlord  to seal any roof  penetration  caused by the
installation,  maintenance or removal of Tenant's  equipment and Tenant shall be
responsible for all roof repairs necessitated by the installation,  maintenance,
use or removal of such  equipment,  including  any roof repairs which would have
been covered by a warranty lost by reason of any of the same.

                  29.2.8  Tenant  shall not,  in the  operation  of any Roof Use
permitted herein,  generate any  electromagnetic  interference with the existing
equipment of Landlord and other tenants (if any) of the  Building,  such as (but
not  limited  to)  electromagnetic  interference  caused by  Tenant's  antennas,
satellite dishes,  cables or other related equipment.  Tenant  acknowledges that
Landlord will not be obligated to modify the Building, any equipment therein (or
on the roof  thereof)  or require any tenant to make  modifications  of existing
equipment in order to accommodate  Tenant's  equipment,  and/or Roof Use, or for
any other purpose. With regard to any new equipment installed by either Landlord
or Tenant in the Building,  each party will install and use such  equipment in a
manner which will avoid  electromagnetic  interference with equipment already in
use by the parties on a "first come, first served" basis.

         29.3     GENERATOR AND GENERATOR FUEL TANK.

                  29.3.1 Subject to (i) compliance with all rules,  regulations,
statutes and codes of any governmental authority having jurisdiction  thereover,
(ii) compliance with any covenants,  conditions and  restrictions  applicable to
the Building,  and (iii)  Landlord's  prior written  consent as to the location,
type of fuel tank and/or  generator,  and method of installation,  which consent
shall not be unreasonably  withheld,  conditioned or delayed,  Tenant shall have
the  right  of  access  to and use of a space  for the  placement  and use of an
emergency  generator  ("Generator")  and  related  above-ground  fuel  tank (the

<PAGE>

"Generator  Fuel  Tank")  pursuant  to  plans  and  specifications  approved  by
Landlord.

                  29.3.2 If the rate of any  insurance  carried by  Landlord  is
increased as a result of Tenant's installation of the Generator and/or Generator
Fuel Tank,  then  Tenant  will pay to  Landlord  within  thirty  (30) days after
Landlord  delivers to Tenant a certified  statement  from  Landlord's  insurance
carrier  stating that the rate increase was caused  thereby,  a sum equal to the
difference  between the original  premium and the  increased  premium  resulting
therefrom.

                  29.3.3 Landlord has not made any  representations  or promises
pertaining  to the  suitability  of the area to be designated on the exterior of
the Building for the  installation  and operation of the Generator and Generator
Fuel  Tank.  Tenant,  for the  purpose of this  paragraph,  agrees to accept the
space(s)  designated by Landlord for such purpose in its then "as is" condition.
The  foregoing  notwithstanding,  Landlord  agrees to  cooperate  with Tenant to
select an area which is reasonably suitable for the operation and maintenance of
the Generator and Generator  Fuel Tank,  provided  Tenant  provides  appropriate
specifications to Landlord prior to such designation to enable Landlord to cause
the designation of such area to be suitable to accommodate such use.

                  29.3.4 Tenant will obtain prior to  installation,  any and all
necessary licenses,  approvals,  permits,  etc., necessary for the installation,
maintenance  and use of any equipment  installed  pursuant to this Section 29.3.
Tenant's installation,  operation, use, removal and replacement of the Generator
and Generator Fuel Tank shall not in any way conflict with any  applicable  law,
statute, ordinance or governmental rules or regulation now in force or which may
hereafter be enacted.  The Tenant will,  at its sole cost and expense,  promptly
comply or ensure that the  connection of Tenant's  Generator and Generator  Fuel
Tank to the Building and Premises,  and the use and operation thereof,  complies
with all laws,  statutes,  ordinances,  governmental  rules or  regulations,  or
requirements of any board of fire insurance underwriters or other similar bodies
now or  hereafter  constituted  relating to or affecting  thereto.  Tenant shall
indemnify  and hold Landlord  harmless  from and against any and all loss,  cost
(including  reasonable  attorney's  fees  incurred by Landlord in enforcing  its
rights  under  this  Section  29.3),  damage  or  liability  arising  out of any
violations of said laws,  statutes,  ordinances rule or regulations,  or arising
out of the use,  operation and  maintenance of said Generator and Generator Fuel
Tank.

                  29.3.5  Tenant's  use  and  operation  of  the  Generator  and
Generator  Fuel Tank shall be  exercised:  (1) in such manner as will not create
any  hazardous  condition  or  interfere  with or impair  the  operation  of the
heating, ventilation, air conditioning,  plumbing,  electrical, fire protection,
life safety,  public  utilities or other  systems or facilities in the Building;
(2) in compliance with all applicable laws, codes and regulations, including any
and all applicable  environmental laws, regulations and legal requirements;  (3)
in such a manner as will not  directly  or  indirectly  interfere  with,  delay,
restrict or impose any expense,  work or obligation  upon Landlord in the use or
operation of such  Building;  and (4) at Tenant's  cost,  including  the cost of
repairing  all damage to the Building and any personal  injury  and/or  property
damage attributable to the installation,  inspection,  adjustment,  maintenance,
removal or  replacement of any  equipment,  apparatus or facilities  pursuant to
this Section 29.3.  Such use shall be confined  solely to the ordinary course of
Tenant's business  operations,  and shall be personal to Tenant and any party to
whom Tenant assigns this Lease pursuant to Section 11.4 hereof.

                  29.3.6 In  connection  with  Tenant's use of the Generator and
Generator Fuel Tank, and subject to the above-stated responsibilities of Tenant,
Tenant shall have the right to operate the  Generator at such  intervals and for
such periods of time as may be recommended by or required by the manufacturer of
such  generator,  or at such other  intervals as Tenant  deems  necessary in its
reasonable  judgment  PROVIDED (i) Tenant will provide notice to Landlord of the
scheduled  times for  regular  testing and  operation,  (ii) Tenant will use all
reasonable  and  diligent  efforts  to  perform  any such  testing  or  periodic
operation  outside of Normal  Business Hours (it being  acknowledged by Landlord
that  certain  testing and  operation  will  necessarily  take place during peak
operational periods,  which may include during Normal Business Hours), and (iii)
such testing will be performed in a manner reasonably calculated

<PAGE>

to minimize any  inconvenience  to other  tenants and occupants of the Building,
and their respective employees and invitees.

                  29.3.7 Tenant  agrees that Landlord  shall have the option (to
be exercised by written  notice to Tenant at or about the  expiration or earlier
termination  of this Lease) to require Tenant to remove  Tenant's  Generator and
Generator Fuel Tank at the expiration or earlier  termination of this Lease,  in
which event Tenant will remove same, repair any damage to the Building caused by
such  removal,  and restore any portion of the  Building  and  appurtenant  site
improvements  which were  affected by the  installation  thereof to its original
condition,  such removal and  restoration  to be completed  within five (5) days
after the later of (i) the date of Landlord's  notice exercising such option, or
(ii) the date of expiration or earlier  termination  of this Lease.  If Landlord
does not  exercise  such  option  within  thirty  (30)  days  after  the date of
expiration or earlier  termination of this Lease, such equipment shall be deemed
to have been  surrendered  with the  Premises,  and Tenant shall have no further
obligation to remove same under this Section 29.3.7.

         29.4     RIGHT OF FIRST OFFER.

                  29.4.1 Subject to the expansion rights of other tenants in the
Building,  if any,  as of the date  hereof  and  described  in EXHIBIT F of this
Lease,  during  the  Lease  Term,  Landlord  agrees  that,  as and  when  (or if
practicable,  within reasonable proximity,  not to exceed [***] months prior, to
the date) space becomes available for lease in the Building, Landlord shall give
notice (an "Offer Notice") to Tenant of the terms upon which Landlord is willing
to lease to Tenant that portion of the Building which Landlord  believes will be
coming available as identified in Landlord's  notice (each such space, an "Offer
Space"),  including (i) a description  of the rentable area of each Offer Space;
(ii) the date on which  Landlord  estimates  in good faith that such Offer Space
will be available for occupancy; (iii) the annual "Base Rent" per square foot of
rentable  area which  Landlord  intends to charge for such space,  including all
fixed  and/or  indexed  adjustments  to said rate (and which shall be based upon
Landlord's  good faith  determination  of the fair market rental and escalations
for such space); (iv) the proposed lease term for such space (and which shall be
based upon Landlord's good faith determination of market terms for the term of a
new lease);  (v) the  condition  the space is proposed to be placed in as of the
commencement  of the  proposed  lease;  and (vi) all other terms which  Landlord
intends to offer with respect to such space,  including  any tenant  improvement
allowance.

                  29.4.2  Tenant  may,  within  [***]  business  days  after the
receipt of the Offer  Notice,  give notice to Landlord  agreeing to lease all of
(but not less  than all of) the  Offer  Space in  accordance  with the terms set
forth in the Offer Notice. If Tenant shall give such notice, then Landlord shall
within thirty (30) days deliver to Tenant a lease  ("Offer  Lease") of the Offer
Space having the terms  specified in the Offer Notice and  otherwise  consistent
with the legal terms of this Lease.

                  29.4.3  Should  Tenant fail to give notice under  Subparagraph
29.4.2  above  within  the time  provided,  Tenant  shall be deemed  not to have
exercised  this Right of First Offer,  and Landlord  shall be free to lease such
Offer Space to a third party; PROVIDED that if Landlord has not executed a lease
for the Offer Space, or any portion thereof, within [***] after such date, or if
Landlord shall  negotiate (or seek to negotiate) a lease for the Offer Space (or
such portion) at a net effective rental rate which is less than [***] of the net
effective rental rate which was set forth in Landlord's most recent Offer Notice
for such Offer Space, Landlord will, prior to executing such lease or continuing
to seek a tenant under such  revised  terms,  provide a revised  Offer Notice to
Tenant with respect to such Offer Space (or portion),  reflecting the applicable
revised  terms,  and Tenant  will have the right,  pursuant  to Section  29.4.2,
above,  to exercise its option  rights under the terms set forth in such revised
Offer  Notice for a period of five (5)  business  days after its receipt of such
revised Offer  Notice.  Should Tenant fail to execute and deliver an Offer Lease
to Landlord  within the [***]  business day period  after  delivery to Tenant of
such Offer Lease reflecting the agreed upon terms,  then this Section 29.4 shall
thereafter be null and void and of no further  force and effect.  Time is of the
essence of this Section 29.4.

                  29.4.4 In connection with Tenant's possible expansion into any
space  leased to a tenant of the  Building  which is proposed to be subleased or
assigned  by such  tenant  during the [***]  year  period in which this Right of
First  Offer is in effect,  Landlord  agrees to notify  Tenant in writing of its
receipt of any  proposed  sublease or  assignment  it receives  from such tenant
during such period (which notice shall also state whether,  and upon what terms,
Landlord has the right to recapture all or any part of the applicable premises).

<PAGE>

         29.5 CONDUIT FOR FIBER OPTIC CABLE. Subject to the provisions hereafter
set forth,  Tenant  shall have the right  during the Term to install on the Land
from a point of  connection  in the public  right of way adjacent to the Land to
that portion of the exterior of the Building  which is  immediately  adjacent to
the Premises,  in a location  approved by Landlord,  a four-inch conduit for the
purpose of  conducting  fiber optic cable from then  existing  fiber optic cable
lines located within such public  right-of-way  to the Premises,  including,  if
necessary,  an  appropriate  manhole for access to such conduit.  Subject to the
Landlord's approval as to complete  engineering plans and specifications and the
manner of  implementation  such  connection,  which  shall  not be  unreasonably
withheld,  Landlord will permit Tenant to conduct such  installation and connect
such conduit and cable to the any applicable telecommunications equipment inside
of the  Premises  or operated  as part of  Tenant's  Roof Use.  Tenant (a) shall
install  such  conduit  and  cable  in  compliance  with all  applicable  codes,
ordinances,  and statutes,  (b) shall be responsible to obtain all  governmental
approvals,  permits,  licenses,  and the like related to such installation,  (c)
shall comply with all legal and insurance  requirements  applicable thereto, (d)
shall repair  and/or  restore any and all areas of Building or Land  affected by
such  installation,  and (e) shall indemnify and hold Landlord  harmless for any
claims,  damages,  losses,   liabilities,   or  expenses  (including  reasonable
attorneys'  fees)  which  Landlord  may  incur  or  sustain  arising  out of the
installation,  use or operation of any  facilities  installed  pursuant  hereto,
including  by reason of any  failure  of  Tenant to comply  with the  provisions
hereof,  to install and operate  such conduit and cable in  compliance  with all
applicable codes,  ordinances,  and statutes and for any damage to the Premises,
Building,  or Land caused by such  installation,  use, operation and/or removal.
Landlord  shall not be liable for the failure of the conduit or cable or for the
improper   installation  thereof  by  Tenant.  Upon  the  expiration  or  sooner
termination of this Lease,  Tenant will, at Landlord's sole election and request
(and at Tenant's  sole  expense),  remove such  conduit and any cable  installed
therein,  along with any  ancillary  equipment  or  structures,  and restore the
Building and Land to its pre-existing  condition (and shall repair any damage to
the  Premises,  the  Building,  or any  other  improvements  to the Land  caused
thereby),  and shall  disconnect  the cable  entirely  from the Premises and the
Building  and repair  any damage to the  Premises,  the  Building,  or any other
improvements to the Land caused thereby. Unless Landlord so elects, Tenant shall
surrender the foregoing  facilities and conduit to be installed pursuant to this
subparagraph  in the  Building  and  Premises  upon  the  expiration  or  sooner
termination of this Lease.

         IN WITNESS WHEREOF,  Landlord and Tenant have executed this Lease as of
the day and year first written above.

                    LANDLORD:

                    11720 Sunrisecorp., L.L.C., a Maryland limited liability
                    company

                    By:    /s/ Victor K. Tolkan
                         ----------------------------------------
                    Name: Victor K. Tolkan
                         ----------------------------------------
                    Title: Managing Member
                         ----------------------------------------

                    TENANT:

                    PATHNET, INC. a Delaware corporation

                    By:    /s/ James M. Craig
                         ----------------------------------------
                    Title: Chief Financial Officer
                         ----------------------------------------EXHIBIT 10.1

                                                                       EXECUTION

                         BALDWIN PIANO & ORGAN COMPANY

                     FIRST AMENDMENT AND LIMITED WAIVER TO
                                CREDIT AGREEMENT

         This FIRST AMENDMENT AND LIMITED WAIVER TO CREDIT AGREEMENT (this
"AMENDMENT") is dated as of August 11, 2000 and entered into by and among
BALDWIN PIANO & ORGAN COMPANY, a Delaware corporation ("BORROWER"), GENERAL
ELECTRIC CAPITAL CORPORATION, as agent for itself and Lenders ("AGENT"), the
financial institutions listed on the signature pages hereof (the "LENDERS"),
and, for purposes of Section 5 hereof, the Credit Parties listed on the
signature pages hereof, and is made with reference to that certain Credit
Agreement dated as of March 24, 2000, by and among Borrower, the other Credit
Parties party thereto, Lenders and Agent, (as modified and amended, the "CREDIT
AGREEMENT"). Capitalized terms used herein without definition shall have the
same meanings herein as set forth in the Credit Agreement.

                                 R E C I T A L S

         WHEREAS, Borrower has requested that Agent and Requisite Lenders waive
any Default or Event of Default which may occur as a result of Borrower's
failure to comply with the Minimum Fixed Charge Coverage and Minimum EBITDA
covenants contained in Annex G of the Credit Agreement as of June 30, 2000, and
(ii) the Minimum Consolidated Tangible Net Worth covenant contained in Annex G
to the Credit Agreement during the period from June 30, 2000 to September 30,
2000;

         WHEREAS, Requisite Lenders and Agent have agreed to waive any such
Default and Event of Default and to make certain amendments to the Credit
Agreement, but only on the terms and conditions hereinafter set forth.

         NOW, THEREFORE, in consideration of the premises and the terms and
conditions herein contained, Borrower, Agent and Requisite Lenders hereby agree
pursuant to subsection 11.2 of the Credit Agreement as follows:

                                   SECTION 1.
                       AMENDMENTS TO THE CREDIT AGREEMENT

         On the basis of the representations and warranties contained in this
Amendment, and subject to the terms and the satisfaction of the conditions set
forth in this Amendment, Agent and Requisite Lenders hereby agree as follows:

         A. AMENDMENT TO SECTION 1.5(a). Section 1.5(a) of the Credit Agreement
is hereby amended by inserting the following paragraph at the end thereof as
follows:

<PAGE>

         "Notwithstanding anything in the foregoing to the contrary, from and
after August 1, 2000, the Applicable Index Margin and Applicable L/C Margin
shall be fixed at 3.00% and 2.50%, respectively, without further adjustment."

         B. AMENDMENT TO ANNEX G. Annex G of the Credit Agreement is hereby
amended by re-lettering the second paragraph (c) thereof (Minimum Tangible Net
Worth) as paragraph (d).

                                   SECTION 2.
                                 LIMITED WAIVER

         On the basis of the representations and warranties contained in this
Amendment, and subject to the terms and conditions of this Amendment, Agent and
Requisite Lenders hereby agree to waive any Default or Event of Default which
may have occurred, or may hereafter occur, under Section 8.1(b) of the Credit
Agreement solely as a result of the Borrower's failure to comply with (i) the
covenants contained in paragraphs (b) (Minimum Fixed Charge Coverage Ratio) and
(c) (Minimum EBITDA) of Annex G to the Credit Agreement as of June 30, 2000 for
the Fiscal Quarter ending on such date and (ii) the covenant contained in
paragraph (d) (Minimum Tangible Net Worth) of Annex G to the Credit Agreement
for the period commencing on June 30, 2000 and continuing to but excluding
September 30, 2000; PROVIDED that, the waivers set forth in this Section 2 shall
cease to be of any force and effect on September 30, 2000.

                                   SECTION 3.
                              LIMITATION OF WAIVER

         Without limiting the generality of the provisions of Section 11.2 of
the Credit Agreement, the waiver set forth above shall be limited precisely as
written and relates solely to noncompliance by Borrower with the provisions of
paragraphs (b), (c), and (d) of Annex G of the Credit Agreement in the manner
and to the extent described above and nothing in this Amendment shall be deemed
to:

         (a) constitute a waiver of compliance by Borrower with respect to (i)
         paragraphs (b), (c), and (d) of Annex G of the Credit Agreement in any
         other instance or (ii) any other term, provision or condition of the
         Credit Agreement or any other instrument or agreement referred to
         therein; or

         (b) prejudice any right or remedy that Agent or any Lender may now have
         (except to the extent such right or remedy was based upon existing
         defaults that will not exist after giving effect to this Amendment) or
         may have in the future under or in connection with the Credit Agreement
         or any other instrument or agreement referred to therein.

         Except as expressly set forth herein, the terms, provisions and
conditions of the Credit Agreement and the other Loan Documents shall remain in
full force and effect and in all other respects are hereby ratified and
confirmed.

                                       2
<PAGE>

                                   SECTION 4.
                         REPRESENTATIONS AND WARRANTIES

         In order to induce Agent and Lenders to enter into this Amendment,
Borrower hereby represents and warrants to Agent and Lenders that:

         A. AUTHORIZATION; BINDING OBLIGATIONS. Each Credit Party has all
requisite corporate power and authority to enter into this Amendment. The
execution, delivery and performance of this Amendment have been duly authorized
by all necessary corporate action by each Credit Party. This Amendment has been
duly executed and delivered by each Credit Party and is the legal, valid and
binding obligation of each Credit Party, enforceable against each Credit Party
in accordance with its terms, except to the extent that the enforceability
thereof may be limited by applicable bankruptcy, insolvency, fraudulent
conveyance, reorganization, moratorium or similar laws generally affecting
creditors' rights and by equitable principles (regardless of whether enforcement
is sought in equity or at law). As of the Amendment Effective Date, the Credit
Agreement, as amended by this Amendment, will constitute the legal, valid and
binding obligation of each Credit Party, enforceable against each Credit Party
in accordance with its terms, except to the extent that the enforceability
thereof may be limited by applicable bankruptcy, insolvency, fraudulent
conveyance, reorganization, moratorium or similar laws generally affecting
creditors' rights and by equitable principles (regardless of whether enforcement
is sought in equity or at law).

         B. INCORPORATION OF REPRESENTATIONS. Each representation and warranty
of each Credit Party contained in each of the Loan Documents is true, correct
and complete in all material respects on and as of the Amendment Effective Date
to the same extent as though made on and as of the Amendment Effective, except
to the extent such representations and warranties relate to an earlier date, in
which case they were true, correct and complete in all material respects as of
such earlier date.

         C. ABSENCE OF DEFAULTS. No event has occurred and is continuing or
would result from the execution, delivery or performance of this Amendment that
constitutes or would constitute a Default or Event of Default after giving
effect to this Amendment.

         D. PERFORMANCE. Borrower has performed in all material respects all
agreements to be performed on its part on or before the date hereof as set forth
in the Credit Agreement.

                                   SECTION 5.
                  ACKNOWLEDGEMENT AND CONSENT BY CREDIT PARTIES

         Borrower and each other Credit Party executing a counterpart hereto
agree to and acknowledge the terms and provisions of this Amendment and confirm
that each Loan Document to which such Credit Party is a party shall continue in
full force and effect and that all of its obligations thereunder shall be valid
and enforceable and shall not be impaired or affected by the execution of this
Amendment, except as specifically provided herein. Borrower and each other
Credit Party executing a counterpart hereof represent and warrant that, all
representations and warranties contained in each Loan Document to which such

                                       3
<PAGE>

Credit Party is a party are true, correct and complete in all material respects
as of the date hereof to the same extent as though made on each such date and
that Borrower and each such Credit Party has performed in all material respects
all agreements to be performed on its part on or before the date hereof as set
forth in the Loan Documents.

                                   SECTION 6.
                           CONDITIONS TO EFFECTIVENESS

         This Amendment shall become effective (the "AMENDMENT EFFECTIVE DATE")
only upon receipt of the following items by Agent:

         (1) counterparts hereof duly executed by each Credit Party and
Requisite Lenders (or, in the case of any Lender, telex or telephone
confirmation from such Lender of its execution hereof); and

         (2) an amendment fee equal to $25,000 payable to Agent for the ratable
benefit of the Lenders.

                                   SECTION 7.
                                  MISCELLANEOUS

         A. EFFECT OF AMENDMENT. Except as specifically provided herein, this
Amendment does not in any way waive, amend, modify, affect or impair the terms
and conditions of the Credit Agreement or the other Loan Documents, and all
terms and conditions of the Credit Agreement and the other Loan Documents are
hereby ratified and confirmed and shall remain in full force and effect unless
otherwise specifically amended, waived, modified or changed pursuant to the
terms and conditions of this Amendment.

         On and after the Amendment Effective Date, each reference in the Credit
Agreement to "this Agreement", "hereunder", "hereof", "herein" or words of like
import referring to the Credit Agreement, and each reference in the other Loan
Documents to the "Credit Agreement", "thereunder", "thereof", or words of like
import referring to the Credit Agreement shall mean and be a reference to the
Credit Agreement as modified by this Amendment.

         B. FEES AND EXPENSES. Borrower acknowledges that all costs, fees and
expenses as described in Section 11.3 of the Credit Agreement incurred by Agent,
Lenders and their respective counsel with respect to this Amendment and the
documents and transactions contemplated hereby shall be for the account of
Borrower.

         C. HEADINGS. Section and subsection headings in this Amendment are
included herein for convenience of reference only and shall not constitute a
part of this Amendment for any other purpose or be given any substantive effect.

         D. APPLICABLE LAW. THIS AMENDMENT SHALL BE GOVERNED BY, AND SHALL BE
CONSTRUED AND ENFORCED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK
APPLICABLE TO CONTRACTS MADE AND PERFORMED IN THAT STATE AND ANY APPLICABLE LAWS
OF THE UNITED STATES OF AMERICA.

                                       4
<PAGE>

         E. COUNTERPARTS. This Amendment may be executed in any number of
counterparts, including by telecopy, and by different parties hereto in separate
counterparts, each of which when so executed and delivered shall be deemed an
original, but all such counterparts together shall constitute but one and the
same instrument; signature pages may be detached from multiple separate
counterparts and attached to a single counterpart so that all signature pages
are physically attached to the same document.

         IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
duly executed and delivered by their respective officers thereunto duly
authorized as of the date first written above.

                                   BORROWER:

                                   BALDWIN PIANO & ORGAN COMPANY

                                   By: /s/ DUANE D. KIMBLE
                                      ------------------------------
                                       Name:   Duane D. Kimble
                                       Title:  Executive Vice President,
                                       Chief Financial Officer and
                                       Secretary

                                   AGENT AND LENDERS:

                                   GENERAL ELECTRIC CAPITAL
                                   CORPORATION, AS AGENT FOR ITSELF
                                   AND LENDERS

                                   By: /s/ CHARLES CHIODO
                                      ------------------------------
                                       Name:  Charles Chiodo
                                       Its Duly Authorized Signatory

                                   OTHER CREDIT PARTIES:

                                   THE WURLITZER COMPANY

                                   By: /s/ DUANE D. KIMBLE
                                      ------------------------------
                                       Name:   Duane D. Kimble
                                       Title:  Executive Vice President,
                                       Chief Financial Officer and
                                       Secretary

                                       S-1
<PAGE>

                                    BALDWIN TRADING COMPANY

                                    By: /s/ DUANE D. KIMBLE
                                       ------------------------------
                                        Name:  Duane D. Kimble
                                        Title: Executive Vice President,
                                        Chief Financial Officer and
                                        Secretary

                                    THE BALDWIN PIANO COMPANY (CANADA)
                                    LIMITED

                                    By: /s/ DUANE D. KIMBLE
                                       ------------------------------
                                        Name:  Duane D. Kimble
                                        Title: Executive Vice President,
                                        Chief Financial Officer and
                                        Secretary

                                      S-2

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