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                        THIRD AMENDMENT TO LEASE BETWEEN

                      SCIENCE PARK DEVELOPMENT CORPORATION

                                       and

                        GENAISSANCE PHARMACEUTICALS, INC.






Date:    June 1, 2000

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         This Third Amendment to Lease (this "Third Amendment") is made and
entered into as of the first day of June, 2000 by and between SCIENCE PARK
DEVELOPMENT CORPORATION, a Connecticut corporation having a principal place of
business at 25 Science Park, Suite 223, New Haven, Connecticut 06511 (herein
referred to as "LANDLORD") and GENAISSANCE PHARMACEUTICALS, INC., a Delaware
corporation having a principal place of business at Five Science Park, New
Haven, Connecticut 06511 (herein referred to as "TENANT").

         WHEREAS, Landlord and Tenant are parties to a certain Lease dated as of
September 15, 1998 (the "ORIGINAL LEASE") as amended by First Amendment to Lease
(the "FIRST AMENDMENT") and Second Amendment to Lease (the "SECOND AMENDMENT"),
each dated December 1, 1999 (collectively, the Original Lease, the First
Amendment and the Second Amendment shall hereinafter be referred to as the
"LEASE");

         WHEREAS, pursuant to the Lease, Tenant leases from Landlord certain
space in the buildings known as Building 5 North and Building 5 South in Science
Park, New Haven, Connecticut;

         WHEREAS, Tenant has requested to lease from Landlord: (i) approximately
6,470 rentable square feet of space on the East wing of the first floor of
Building 5 North as shown on the floor plan attached hereto as EXHIBIT A-1; (ii)
approximately 9,112 rentable square feet of space on the East wing of the first
floor of Building 5 South as shown on the floor plan attached hereto as EXHIBIT
A-1 (the space referred to in clauses (i) and (ii) above is herein referred to
collectively as the "1ST FLOOR EAST WING SPACE"); (iii) approximately 4,083
rentable square feet of space on the West wing of the third floor of Building 5
North as shown on the floor plan attached hereto as EXHIBIT A-2 (herein referred
to as the "PENTHOUSE"); (iv) 185 rentable square on the third floor of Building
5 South as shown on the floor plan attached hereto as EXHIBIT A-3 (the "THIRD
FLOOR SPACE"); and (v) the walkway connector between the Building 5 North 1st
Floor East Wing Space and the Building 5 South 1st Floor East Wing Space, if it
is built by Tenant, as shown on the floor plan attached hereto as EXHIBIT A-1
(herein referred to as the "FIRST FLOOR WALKWAY") (collectively, the 1st Floor
East Wing Space, the Penthouse, the Third Floor Space and the First Floor
Walkway shall herein be referred to as the "ADDITIONAL SPACE"; the Additional
Space, excluding the First Floor Walkway, is herein referred to as the "EXISTING
ADDITIONAL SPACE"), upon and subject to the terms, covenants and conditions
contained in the Lease as modified by this Third Amendment; and

         WHEREAS, Landlord and Tenant desire to amend the Lease in certain
respects;

         NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which is acknowledged by each of the parties, Landlord and Tenant
hereby agree as follows (capitalized terms used herein which are not otherwise
defined herein shall have the meaning given to such terms in the Lease):


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1.       REPRESENTATION AS TO CONDITION OF EXISTING ADDITIONAL SPACE; DELIVERY
         OF POSSESSION OF EXISTING ADDITIONAL SPACE. Landlord represents that
         Landlord has (i) caused to be removed from the Existing Additional
         Space all asbestos, which removal was performed in compliance with all
         applicable Environmental Laws and Title 19a ("Public Health and
         Well-Being") of the Connecticut General Statutes, including but not
         limited to Sections 19c-332 through 332e, concerning asbestos, and any
         regulations promulgated thereunder, (ii) caused to be removed from the
         Existing Additional Space all lead-based paint, which removal has been
         performed in compliance with applicable laws, such that as a result of
         the removal, no occupants of Building 5 North or South were exposed to
         airborne lead at levels over the action level, as defined in 29 CFR
         Section 1910.1025, and (iii) otherwise prepared the Existing Additional
         Space for delivery of same to Tenant in a vacant, broom clean
         condition, free of all furnishings, equipment, litter and debris.
         Subject to the foregoing representation, upon the execution and
         delivery of this Third Amendment, Landlord shall deliver to Tenant and
         Tenant shall accept from Landlord possession of the Existing Additional
         Space in its present as-is condition.

2.       REMOVAL OF SHED. The parties acknowledge and agree that, Landlord, at
         its expense, has (i) obtained required demolition permits and
         demolished that certain "shed" structure located in the intended area
         of the First Floor Walkway (the "Shed") , (ii) caused to be removed
         from the Shed prior to its demolition all asbestos, which removal was,
         to the best knowledge of Landlord, performed in compliance with all
         applicable Environmental Laws and Title 19a ("Public Health and
         Well-Being") of the Connecticut General Statutes, including but not
         limited to Sections 19c-332 through 332e, concerning asbestos, and any
         regulations promulgated thereunder, (iii) caused to be removed from the
         Shed all lead-based paint, which removal was, to the best knowledge of
         Landlord, performed in compliance with applicable laws, such that as a
         result of the removal no occupants of Building 5 North or South have
         been exposed to airborne lead at levels over the action level, as
         defined in 29 CFR Section 1910.1025.

3.       THIRD FLOOR SPACE.

         A.       The text of Section 2 of the Second Amendment is hereby
                  deleted and substituted therefor is the following:

                  "The rentable area of the Third Floor Space on the third floor
                  of Building 5 South is deemed to be 185 rentable square feet.
                  Tenant shall have exclusive possession of the Third Floor
                  Space."

         B.       The fourth sentence of Section 7 of the Second Amendment is
                  hereby deleted.

         C.       The Third Floor Space (as defined herein) is contiguous to the
                  Third Floor Space (as defined in the Second Amendment),
                  however, for purposes of the Lease and this Amendment,
                  including without limitation, for purposes of calculating all
                  rentable obligations under the Second Amendment and under this
                  Third Amendment, they shall be treated as separate spaces
                  constituting 185 rentable square feet each.


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4.       DEFINITION OF INTERIOR WORK. Pursuant to the provisions of Sections
         2.4B, 2.4E - H and 2.6 of the Original Lease, Landlord at its expense
         shall undertake all of the Work described on EXHIBIT B-1 hereof (the
         "INTERIOR WORK"). Landlord hereby agrees to complete the Interior Work
         in a good and workmanlike fashion, otherwise Tenant relies on no
         warranties or representations, express or implied, of Landlord or any
         agent or other party associated with Landlord as to its condition or
         repair, or as to taxes or any other matter relating to the Additional
         Space, except as otherwise expressly provided in the Lease, as modified
         by this Third Amendment. In addition, Landlord's representation
         regarding compliance with laws set forth in the second sentence of
         Section 8.1 of the Original Lease is hereby deemed repeated with regard
         to the Interior Work, as is the limitation on Tenant's remedy set forth
         in the balance of said Section 8.1.

5.       INTERIOR WORK.

         A.       Landlord, at its expense, shall use its best efforts to
                  substantially complete the Interior Work so it does not delay
                  Tenant's completion of the Tenant's Work (as hereinafter
                  defined), but in any event, no later than four (4) months
                  following the Existing Additional Space Lease Commencement
                  Date (as hereinafter defined). In addition, Landlord
                  represents that all electrical, mechanical, plumbing and other
                  building systems serving the Additional Space will be in
                  working order on the Additional Space Rent Commencement Date
                  (as hereinafter defined).

         B.       Landlord and Tenant shall each use its best efforts to
                  collectively cause Fusco Corporation and Svigals Associates to
                  coordinate the Interior Work with the Tenant's Work (as
                  defined in Section 15 of this Third Amendment) with respect to
                  the Additional Space in order not to delay completion of the
                  Tenant's Work with respect to the Additional Space.

6.       EXISTING ADDITIONAL SPACE LEASE COMMENCEMENT DATE. Landlord hereby
         agrees to lease to Tenant and Tenant hereby agrees to lease from
         Landlord the Additional Space, upon and subject to the terms of the
         Lease, as modified by this Third Amendment. Landlord shall deliver, and
         Tenant shall accept, exclusive possession of the Existing Additional
         Space on the date of this Third Amendment first written above, which
         date shall be deemed to be the Lease Commencement Date with respect to
         the Existing Additional Space (the "EXISTING ADDITIONAL SPACE LEASE
         COMMENCEMENT DATE"). Landlord's delivery of exclusive possession shall
         be subject to entries by the Landlord and its contractors into the 1st
         Floor East Wing Space and the Third Floor Space only as necessary to
         complete the Interior Work prior to the date Tenant first occupies such
         portions of the Additional Space for its business purposes (the
         "OCCUPANCY DATE"). Because Tenant will not have exclusive possession of
         the 1st Floor East Wing Space or the Third Floor Space until the
         Occupancy Date, the parties hereby agree that the provisions of
         Sections 5.1, 7.2, 7.4, 7.5, 12,1E and 23.1 of the Original Lease shall
         not apply to Tenant with regard to the 1st Floor East Wing Space or the
         Third Floor Space until the Occupancy Date.


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7.       EXISTING ADDITIONAL SPACE. Commencing on the Existing Additional Space
         Lease Commencement Date, the Leased Premises shall be deemed to include
         the Existing Additional Space for all purposes under the Lease, and the
         terms, covenants and conditions of the Lease, as modified by this Third
         Amendment, shall govern the rights, obligations and liabilities of
         Landlord and Tenant with respect to the Existing Additional Space. The
         Leased Premises, as described in Section 1.1(a) of the Original Lease
         and as shown on Exhibits A-1 and A-2 of the Original Lease, are
         sometimes referred to in this Third Amendment as the "ORIGINAL LEASED
         PREMISES".

8.       CONFIRMATION OF TERMS. The Expiration Date of the initial 5-year Term
         of the Lease, as hereby amended, with respect to the entire Leased
         Premises, including without limitation, the Additional Space, is
         February 29, 2004. The date February 28, 2004 as it appears in the
         first sentence of Section 5 of the First Amendment and in the first
         sentence of Section 9 of the Second Amendment is hereby deleted and
         "February 29, 2004" is substituted therefor in each sentence. The two
         5-year Extension Options granted in Section 2.2 of the Original Lease
         shall apply to the entire Leased Premises, including without
         limitation, the Additional Space. The First Amendment Space Lease
         Commencement Date (as defined in the First Amendment) occurred on
         January 14, 2000, and the Additional Space Lease Commencement Date (as
         defined in the Second Amendment) occurred on February 7, 2000. Tenant
         may not exercise an Extension Option with respect to less than the
         entire Leased Premises.

9.       RENT COMMENCEMENT DATE.

         A.       1ST FLOOR EAST WING SPACE. Subject to extensions pursuant to
                  Sections 2.3B. and 2.3C. of the Original Lease, the Rent
                  Commencement Date with respect to the 1st Floor East Wing
                  Space and the Third Floor Space (the "1ST FLOOR EAST WING RENT
                  COMMENCEMENT DATE ") shall mean the earlier of: (i) the date
                  of the issuance of a temporary or permanent certificate of
                  occupancy for all or any portion of the 1st Floor East Wing
                  Space and the Third Floor Space, or (ii) four (4) months after
                  the date of this Third Amendment first written above.

         B.       PENTHOUSE. Subject to extensions pursuant to Sections 2.3B.
                  and 2.3C. of the Original Lease, the Rent Commencement Date
                  with respect to the Penthouse (the "PENTHOUSE RENT
                  COMMENCEMENT DATE") shall mean May 9, 2000.

         C.       INTERPRETATION. In interpreting the provisions of Sections
                  2.3B and 2.3C of the Lease to this Third Amendment, the term,
                  "Interior Work" as used therein shall mean "Interior Work" as
                  defined herein, and the term, "Rent Commencement Date" as used
                  therein shall mean (i) with respect to the 1st Floor East Wing
                  Space and the Third Floor Space, the "1st Floor East Wing Rent
                  Commencement Date", and (ii) with respect to the Penthouse,
                  the "Penthouse Rent Commencement Date", as defined herein. At
                  the request of either Landlord or Tenant, Landlord and Tenant
                  shall execute and deliver to each other a writing confirming
                  the First Floor Walkway Rent Commencement Date (as hereinafter
                  defined), the 1st Floor East Wing Rent Commencement Date, and
                  the Penthouse Rent Commencement Date.


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10.      RENT.

         A.       Because Tenant will bear the cost of building the First Floor
                  Walkway if it is built, Tenant shall pay Additional Rent, but
                  shall have no obligation to pay Base Rent, for the First Floor
                  Walkway during the Term or any Extension Term. Tenant's
                  obligation to pay Additional Rent with respect to the First
                  Floor Walkway shall commence on the date of the issuance of a
                  temporary or permanent certificate of occupancy for the First
                  Floor Walkway ("FIRST FLOOR WALKWAY RENT COMMENCEMENT DATE"),
                  and continue thereafter until the Expiration Date of the
                  Lease.

         B.       1ST FLOOR EAST WING SPACE BASE RENT - INITIAL TERM. In
                  accordance with the provisions of this Third Amendment, Tenant
                  shall pay to Landlord Base Rent with respect to the 1st Floor
                  East Wing Space and the Third Floor Space at the rate of Nine
                  Dollars and Fifty Cents ($9.50) per rentable square foot of
                  the 1st Floor East Wing Space per annum commencing on the 1st
                  Floor East Wing Rent Commencement Date, and continuing
                  thereafter until February 29, 2004. Landlord and Tenant hereby
                  stipulate to the rentable area of the 1st Floor East Wing
                  Space as set forth in the third Whereas clause to this Third
                  Amendment.

                  PENTHOUSE BASE RENT - INITIAL TERM. In accordance with the
                  provisions of this Third Amendment, Tenant shall pay to
                  Landlord Base Rent with respect to the Penthouse at the rate
                  of Fourteen and 56/100 Dollars ($14.56) per rentable square
                  foot of the Penthouse per annum commencing on the Penthouse
                  Rent Commencement Date, and continuing thereafter until
                  February 29, 2004. Landlord and Tenant hereby stipulate to the
                  rentable area of the Penthouse as set forth in the third
                  Whereas clause to this Third Amendment.

                  BASE RENT - EXTENSION TERMS. Notwithstanding Section 3.1C. of
                  the Original Lease or anything else to the contrary contained
                  in the Lease, during the first 5-year Extension Term,
                  commencing on March 1, 2004 and ending on February 28, 2009,
                  if any, and the second 5-year Extension Term (commencing on
                  March 1, 2009 and ending on February 28, 2014), if any,
                  respectively, with respect to the Existing Additional Space
                  only, Tenant shall pay to Landlord a Base Rent equal to ninety
                  percent (90%) of the then fair market value rent for the
                  Existing Additional Space as determined by mutual agreement
                  between the parties, or if the parties fail to agree, as shall
                  be determined by arbitration in accordance with the Rules of
                  Commercial Arbitration of the American Arbitration Association
                  then in effect. In the event arbitration is necessary,
                  Landlord and Tenant shall each select one arbitrator, each of
                  whom shall be a commercial real estate broker with at least
                  ten (10) years of experience in the New Haven commercial real
                  estate market. If the two arbitrators are unable to agree on
                  the fair market value rent within fifteen days of their
                  selection, they shall appoint a third arbitrator and the
                  determinations of fair market value rent by the third
                  arbitrator so selected shall be final. The decision of the
                  arbitrators shall be binding on the parties. Each party shall
                  pay the fees and expenses of the arbitrator it selects and
                  the fees and


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                  expenses of the third arbitrator, if one is necessary, shall
                  be shared equally by the parties.

         C.       ADDITIONAL RENT. Notwithstanding anything to the contrary
                  contained herein, for purposes of calculating each and every
                  instance where Additional Rent is payable hereunder with
                  regard to the Additional Space, the Additional Space shall be
                  deemed to include 100% of the rentable area of the Existing
                  Additional Space and the First Floor Walkway, if it is built,
                  as determined in accordance with the provisions of Section 16
                  hereof. With respect to: (i) the 1st Floor East Wing Space and
                  the Third Floor Space, commencing on the 1st Floor East Wing
                  Space Rent Commencement Date, (ii) the Penthouse, commencing
                  on the Penthouse Rent Commencement Date, and (iii) the First
                  Floor Walkway, commencing on the First Floor Walkway Rent
                  Commencement Date, and, in each case, continuing thereafter
                  until the Expiration Date of the Lease, Tenant shall: (i) pay
                  for electricity and gas consumed in the portion of the
                  Additional Space in question in accordance with Section 3.4 of
                  the Original Lease, (ii) pay Additional Rent with respect to
                  the portion of the Additional Space in question in accordance
                  with Section 3.5 of the Original Lease, as modified by this
                  Third Amendment.

         D.       OPERATING EXPENSE PAYMENTS. In addition to Base Rent and other
                  Additional Rent due and payable by Tenant pursuant to the
                  other provisions of this Lease, Tenant shall pay to Landlord,
                  as Additional Rent, commencing on the 1st Floor East Wing Rent
                  Commencement Date (with respect to the 1st Floor East Wing
                  Space and the Third Floor Space), and commencing on the First
                  Floor Walkway Rent Commencement Date (with respect to the
                  First Floor Walkway), an amount equal to Tenant's OE Share (as
                  herein defined) of Operating Expenses paid or incurred by
                  Landlord during the Term of this Lease or any Extension Term,
                  regardless of whether any Operating Expense Year occurs in
                  whole or in part during the Term or any Extension Term;
                  provided, however, that for purposes of calculations made
                  under this Paragraph 10.D., in no event shall Landlord charge
                  Tenant more than Tenant's OE Share of 100% of the actual cost
                  of Operating Expenses incurred by Landlord.

                  ESTIMATED OE PAYMENTS. Additional Rent due and payable by
                  Tenant to Landlord pursuant to this Paragraph 10. D. shall be
                  paid in the following manner:

                           (a)      Landlord shall reasonably estimate in
                  advance the amounts Tenant shall owe under this Article for
                  any full or partial Operating Expense Year. Beginning in the
                  first Operating Expense Year, Tenant shall pay such estimated
                  amounts, on a monthly basis, on or before the first day of
                  each calendar month during the first and each following
                  Operating Expense Year. Such estimate may be reasonably
                  adjusted from time to time by Landlord.

                           (b)      Within 120 days after the end of each
                  Operating Expense Year, or as soon thereafter as practicable,
                  Landlord shall provide a statement (the "STATEMENT") to Tenant
                  showing: (i) the amount of actual Operating Expenses


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                  for such Operating Expense Year, with a listing of amounts for
                  major categories of Operating Expenses, (ii) any amount paid
                  by Tenant towards Additional Rent payable under this Article
                  during such Operating Expense Year on an estimated basis,
                  (iii) any revised estimate of Tenant's obligations for
                  Operating Expenses for the current Operating Expense Year, and
                  (iv) a calculation of whether the Operating Expense Cap has
                  been exceeded.

                           (c)      If the Statement shows that the estimated
                  payments made by Tenant during the Operating Expense Year were
                  less than Tenant's actual obligations for the payment of
                  Additional Rent under this Paragraph 10. D. for such Year,
                  Tenant shall pay the difference. If the Statement shows an
                  increase in Tenant's estimated payments for the current
                  Operating Expense Year, Tenant shall pay the difference
                  between the new and former estimates, for the period from the
                  first day of the then-current Operating Expense Year through
                  the month in which the Statement is sent. Tenant shall make
                  such payments within thirty (30) days after the Statement is
                  deemed to have been given under the terms of this Lease.

                           (d)      If the Statement shows that Tenant's
                  estimated payments exceeded Tenant's actual obligations for
                  the payment of Operating Expenses under this Paragraph 10. D.,
                  then Tenant shall receive a credit for the difference against
                  payments of rent next due. If the Term shall have expired and
                  no further rent shall be due, Tenant shall receive a refund of
                  such difference, within sixty (60) days after Landlord sends
                  the Statement.

                           (e)      No delay by Landlord of nine (9) months or
                  less in providing the Statement (or separate statements) shall
                  be deemed a default by Landlord or a waiver of Landlord's
                  right to require payment of Tenant's obligations under this
                  Article. After such nine (9) month period, Landlord shall be
                  estopped from billing for past Operating Expenses.

                  BOOKKEEPING AND AUDITING. Landlord shall maintain records
                  respecting Operating Expenses and determine the same in
                  accordance with sound accounting and management practices,
                  consistently applied. Tenant or its representative shall have
                  the right to examine those books and records of Landlord and
                  any managing agent reasonably necessary for purposes of
                  auditing the Statement in question, provided Tenant gives
                  Landlord reasonable prior notice specifying the particular
                  respects in which the Statement is claimed to be incorrect.
                  Such examination shall take place during normal business hours
                  at the place or places where such records are normally kept
                  with forty-five (45) days following such notice from Tenant.
                  Each Statement shall be considered final, except as to matters
                  to which exception is taken after examination of Landlord's
                  records in the foregoing manner and within the foregoing
                  times. If Tenant takes exception to any matter contained in
                  the Statement as provided herein, Landlord shall refer the
                  matter to an independent certified public accounting firm
                  having a national practice, whose certification as to the
                  proper amount shall be final and conclusive as between


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                  Landlord and Tenant. Tenant shall promptly pay the cost of
                  such certification unless such certification determines that
                  Tenant was overbilled by more than two (2%) percent. Pending
                  resolution of any such exceptions in the foregoing manner,
                  Tenant shall continue paying, without prejudice to Tenant's
                  position, Tenant's OE Share of Operating Expenses paid or
                  incurred during the applicable Operating Expense Year in the
                  amounts determined by Landlord, subject to adjustment after
                  any such exceptions are resolved.

                  SURVIVAL. The parties' obligations set forth in this Paragraph
                  10. D shall survive the expiration or earlier termination of
                  this Lease for a period of one year, but such survival shall
                  not be deemed to effect the waiver set forth in subparagraph
                  10.D(e) hereof.

                  For purposes of this Paragraph 10.D., "Operating Expenses"
                  shall have the meaning given to such term in Paragraph 37.1
                  (e) of the Lease, as qualified by Paragraph 37.1 (f) of the
                  Lease. The Operating Expense Cap, as defined in Paragraph 37.1
                  (h) of the Lease, shall not apply to the 1st Floor East Wing
                  Space, the Third Floor Space or the First Floor Walkway (if it
                  is built), as defined herein.

                  For purposes of this Paragraph 10.D., "Building" shall have
                  the meaning given to such term in Paragraph 37.1 (c) of the
                  Lease.

                  For purposes of this Paragraph 10.D., "Operating Expense Year"
                  shall mean calendar year 2000 and each calendar year
                  thereafter any portion of which occurs during the Term of the
                  Lease. Notwithstanding the foregoing, the first Operating
                  Expense Year shall commence on the 1st Floor East Wing Rent
                  Commencement Date (with respect to the 1st Floor East Wing
                  Space and the Third Floor Space), and shall commence on the
                  First Floor Walkway Rent Commencement Date (with respect to
                  the First Floor Walkway) and shall end on December 31, 2000,
                  and the last Operating Expense Year shall end on the last day
                  of the Term or the last day of any Extension Term. Tenant's
                  obligation under this Paragraph 10.D. to pay estimated and
                  actual amounts towards Operating Expenses for the first and
                  last Operating Expense Year shall be prorated by multiplying
                  the total estimated or actual (as the case may be) Operating
                  Expenses paid or incurred during the first or last (as the
                  case may be) Operating Expense Year, by a fraction, the
                  numerator of which shall be the number of days in the first or
                  last (as the case may be) Operating Expense Year, and the
                  denominator of which shall be 365.

                  For purposes of this Paragraph 10.D., "Tenant's OE Share"
                  shall mean a fraction, the numerator of which shall be the
                  total number of rentable square feet in the First Floor East
                  Wing Space plus the Third Floor Space (185 rentable square
                  feet) and the denominator of which shall be the total number
                  of rentable square feet in the Building. If the First Floor
                  Walkway is built, the rentable square footage of the Building
                  and the 1st Floor East Wing Space shall be deemed for purposes
                  of calculating Tenant's OE Share to include the rentable
                  square footage of the First Floor Walkway, as the case may be.


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                  For purposes of this Paragraph 10.D., "Park Expenses" shall
                  have the meaning given to such term in Paragraph 37.1 (j) of
                  the Lease.

                  GROSS UP PROVISION. If the Building is not fully occupied
                  during all or a portion of any Operating Expense Year,
                  Landlord may, in accordance with sound accounting and
                  management practices, determine the amount of Operating
                  Expenses actually paid or incurred by Landlord that would have
                  been included in Operating Expenses had the Building been
                  fully occupied and the amount so determined shall be deemed to
                  have been the amount of Operating Expenses for such year,
                  subject to the first sentence of this Section 10D.

         E.       OPERATING EXPENSES FOR PENTHOUSE.
                  Tenant's obligation to pay Operating Expenses for the
                  Penthouse shall be governed by the provisions of Section 17 of
                  the Second Amendment except that:

                           (a)      With respect to the Penthouse, "BASE EXPENSE
                  YEAR" shall mean the twelve (12) month period commencing on
                  the Penthouse Rent Commencement Date.

                           (b)      With regard to the Penthouse, "TENANT'S OE
                  SHARE" shall mean a fraction, the numerator of which shall be
                  the total of the rentable area of the Penthouse and the
                  denominator of which shall be the total number of rentable
                  square feet in the Building 5 North and Building 5 South, plus
                  the rentable area of the Bridge and/or the First Floor
                  Walkway, if either is built.

                           (c)      Tenant's obligation to begin paying
                  Operating Expenses with respect to the Penthouse shall not
                  commence until the first Operating Expense Year occurring
                  immediately following the Base Expense Year attributable to
                  the Penthouse.

                           (d)      On or before the first day of each Operating
                  Expense Year following the Base Expense Year attributable to
                  the Penthouse, Landlord shall give Tenant statements of the
                  estimated amount of increase in Operating Expenses that Tenant
                  will owe with respect to such space for ensuing Operating
                  Expense Year, and Tenant shall make payments monthly in
                  accordance with such statement.

11.      DEFINITIONS. The first sentence of Section 4.1(d) of the Original
         Lease, Section 8(b) of the First Amendment and Paragraph 12 of the
         Second Amendment are hereby deleted and substituted therefor is the
         following:

                  "Tenant's Pro-Rata Share" shall mean a fraction, the numerator
                  of which shall be the total number of rentable square feet in
                  the Leased Premises (taking into account that the rentable
                  area of the Additional Space, as defined in the Second


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                  Amendment, shall be calculated in accordance with the
                  provisions of Section 11(b) of the Second Amendment and the
                  Additional Space, as defined in the Third Amendment, shall be
                  calculated in accordance with the provisions of Paragraph 16
                  of the Third Amendment), as the same may increase or decrease
                  from time to time, and the denominator of which shall be the
                  total number of rentable square feet in Building 5 North and
                  Building 5 South combined, as same may increase or decrease
                  from time to time, plus (i) the rentable area of the First
                  Floor Walkway, if it is built, and (ii) the rentable area of
                  the Bridge (as such term is defined in the Second Amendment),
                  if it is built; Landlord represents that as of the date hereof
                  the combined area of Building 5 North and Building 5 South,
                  excluding the Bridge and the First Floor Walkway, measured in
                  rentable square feet is approximately 102,938 rentable square
                  feet.

12.      TAX PAYMENT.

         A.       Commencing on the 1st Floor East Wing Rent Commencement Date,
                  Paragraph 13 of the Second Amendment is hereby deleted and
                  substituted therefor is the following:

                  "Commencing on the 1st Floor East Wing Rent Commencement Date
                  (as defined in the Third Amendment), Tenant shall pay to
                  Landlord as Additional Rent due hereunder, for any Tax Year,
                  any part of which occurs after the 1st Floor East Wing Rent
                  Commencement Date and during the Term or any Extension Term,
                  an amount (the "TAX PAYMENT") equal to: (i) Tenant's Pro-Rata
                  Share (as defined in the Third Amendment) of the Taxes
                  attributable to the Base Building, plus (ii) 100% of the Taxes
                  attributable to Tenant's Work (which for purposes of the
                  balance of this Article 4 shall mean collectively Tenant's
                  Work as defined under the Original Lease, the First Amendment,
                  the Second Amendment and the Third Amendment)."

         B.       It is the parties' intent that the provisions of Sections
                  9(a)(iii), 9(b) and 9(c) of the First Amendment apply
                  generally to the Second Amendment and this Third Amendment and
                  govern specifically the parties' rights and obligations with
                  respect to Tax Payments, Enterprise Zone benefits and
                  apportionment of both hereunder.

13.      PARKING. The rental of the Additional Space will include the use of
         forty-seven (47) parking spaces in addition to the eighty (80) parking
         spaces provided for under the Original Lease, the twenty-five (25)
         parking spaces provided for under the First Amendment and the fifty
         (50) parking spaces provided for under the Second Amendment. The term
         "TENANT'S PARKING SPACES" shall be amended to include such additional
         forty-seven (47) parking spaces, bringing the total of Tenant's Parking
         Spaces to two hundred and two (202). Ten (10) of said additional
         forty-seven (47) parking spaces shall be made available to Tenant
         upon the execution and delivery of this Third Amendment, twenty (20) of
         said additional forty-seven (47) parking spaces shall be made available
         to Tenant

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<PAGE>


         as of the date Tenant takes occupancy of the 1st Floor East Wing
         Space for the purpose of conducting its business therein, and the
         remaining seventeen (17) parking spaces shall be made available to
         Tenant as of the date which is six (6) months after the date of this
         Third Amendment first written above. All of Tenant's Parking Spaces
         shall be located within the areas shown on EXHIBIT A-4 at all times
         during the Term or any Extension Term.

14.      AS-IS CONDITION. Subject to completion of the Landlord's obligations
         set forth in this Third Amendment, including without limitation,
         removal of the Shed and completion of the Interior Work, Landlord shall
         tender Tenant possession of the Additional Space in its then "as is"
         condition and Tenant agrees to accept possession of the Additional
         Space in its then "as is" condition, broom clean.

15.      TENANT'S WORK. Tenant, at Tenant's expense, agrees to make improvements
         to the Additional Space pursuant to plans and specifications approved
         by Landlord. Such improvements shall be deemed "TENANT'S WORK" and such
         plans and specifications shall be deemed "TENANT'S PLANS", and the
         terms of Section 2.7(a) of the Original Lease shall govern the approval
         of same. It is currently Tenant's intention that the Tenant's Work will
         include the construction of the First Floor Walkway. Tenant shall not
         be penalized in any manner if Tenant does not build the First Floor
         Walkway as part of the Tenant's Work. Tenant shall proceed with
         reasonable due diligence to perform Tenant's Work following funding of
         Tenant's new loan from Connecticut Innovations, Inc. (the "NEW CII
         LOAN") and the issuance of a building permit therefor.

16.      THE FIRST FLOOR WALKWAY. If Tenant builds the First Floor Walkway, then
         for all purposes under the Lease, the First Floor Walkway shall be
         deemed a part of the Leased Premises, except that Tenant shall not be
         required to pay Base Rent with respect to the First Floor Walkway. The
         rentable area of the First Floor Walkway shall be remeasured by Svigals
         Associates at Tenant's expense promptly after construction and included
         as part of the Leased Premises for purposes of calculating Tenant's
         Pro-Rata Share and for purposes of calculating Tenant's OE Share (as
         both terms are defined in this Third Amendment). The parties agree that
         the usable area and the rentable area of the First Floor Walkway will
         be the same. Similarly, if the First Floor Walkway is not constructed,
         the rentable area of the Leased Premises for purposes of calculating
         Additional Rent shall not include the rentable area of the First Floor
         Walkway. Tenant shall have no obligation to remove or restore the First
         Floor Walkway upon surrender of the Leased Premises to Landlord,
         notwithstanding the provisions of Section 6.2 of the Original Lease.

17.      LOBBY AND BATHROOMS. For all purposes under the Lease, the lobby and
         bathrooms between the Building 5 North 1st Floor East Wing Space and
         the Building 5 South 1st Floor East Wing Space shall not be deemed a
         part of the Additional Space and have not been and shall not be
         included in the calculation of Base Rent Tenant is obligated to pay
         hereunder. In addition, the rentable square footage of the said lobby
         and bathrooms shall not be included in the calculation of Tenant's
         Pro-Rata Share of Taxes under Article 4 of the Lease, as amended by
         this Third Amendment, and for purposes of calculating Tenant's OE Share
         as defined in Section 12 hereof.


                                      11
<PAGE>

18.      SUBORDINATION AND NON-DISTURBANCE. As used herein, the term "LANDLORD'S
         LENDER" shall mean and refer to the Connecticut Housing Finance
         Authority ("CHFA"), the Connecticut Development Authority ("CDA") and
         each future lender who may from time to time extend credit to Landlord
         which extensions of credit may be secured in whole or in part by a
         mortgage, deed of trust, ground lease or other security interest
         affecting the real property of which the Additional Space is a part.
         Tenant's obligations under this Third Amendment are subject to receipt
         of non-disturbance agreements from CDA and CHFA, both of which shall be
         substantially similar in form and content to the form of
         Non-Disturbance, Subordination and Attornment received from CDA and
         CHFA in connection with the execution of the First Amendment and the
         Second Amendment. Provided CHFA and CDA execute and deliver said
         non-disturbance agreements, the Lease, as modified by this Third
         Amendment, shall be subject and subordinated to: (a) all security
         interests in favor of CHFA and CDA affecting the Leased Premises,
         including without limitation the Additional Space, or the property of
         which the Leased Premises are a part, and (b) all present and future
         mortgages, deeds of trust and other security interests, including
         leasehold mortgages, granted by Landlord in favor of CHFA and CDA and
         affecting the Leased Premises, including without limitation the
         Additional Space, or the property of which the Leased Premises are a
         part. Tenant agrees to execute, at no expense to Landlord, any
         instrument which may reasonably be deemed necessary or desirable by
         Landlord, CHFA or CDA or to further effect the subordination of the
         Lease, as modified by this Third Amendment to Lease, to any such
         security interest, provided however, Tenant's non-disturbance rights
         are not affected. The Tenant's failure to strictly comply with this
         section will constitute an Event of Default under this Lease.

19.      NEW CII LOAN.

         A.       Section 15.1B of the Original Lease is hereby modified to
                  permit Tenant to assign its right, title and interest in and
                  to this Third Amendment without Landlord's consent to CII as
                  collateral security for the New CII Loan, or to any Tenant's
                  Lender as collateral security for a similar loan to Tenant.
                  Tenant's Lender, including, without limitation, CII shall be
                  entitled to assign its rights under this Third Amendment or to
                  sublet the Additional Space to any entity, subject to the
                  provisions set forth in subsections 15.1B(a)-(d) of the
                  Original Lease.

         B.       Upon at least five (5) business days notice, Landlord, at its
                  expense, hereby agrees to enter into a new Consent and
                  Intercreditor Agreement in form and content acceptable to CDA,
                  CII, CHFA and Landlord but relating to the New CII Loan and
                  this Third Amendment. Landlord hereby agrees to work with the
                  CDA, CII and CHFA to facilitate full execution of the new
                  Consent and Intercreditor Agreement by Landlord's Lenders. In
                  addition, Landlord hereby agrees that it will not raise
                  objection to any provisions in the new Consent and
                  Intercreditor Agreement that are substantially similar to the
                  original or amended Consent and Intercreditor Agreements
                  signed in connection with the Original Lease and the First and
                  Second Amendments, respectively.


                                       12
<PAGE>

         C.       Upon the closing of the New CII Loan, Landlord agrees to
                  execute and deliver an affidavit (a) verifying the
                  nonexistence of any tenants' rights in the Additional Space,
                  (b) verifying the nonexistence of any security interests in
                  personal property and fixtures that form a part of the
                  Additional Space, other than the rights therein, if any, of
                  the CDA and CHFA as the current Landlord's Lenders, and United
                  States Department of Commerce, Economic Development
                  Administration and (c) that Landlord has no notice of any
                  facts or circumstances not of record which could give rise to
                  the claim of any third party to rights of adverse possession
                  or use over the Additional Space or any part thereof in
                  derogation of Landlord's title. Upon the execution of this
                  Third Amendment, Landlord further agrees to obtain and deliver
                  a subordination of mechanic's liens, subordinated as to the
                  lien in favor of CII, executed by any contractor(s) who have
                  furnished any labor, services or materials in connection with
                  construction or repair work to the Building on behalf of
                  Landlord and would have a right to file a mechanic's lien that
                  would have priority over the mortgage filed in connection with
                  the New CII Loan.

20.      AMENDED NOTICE OF LEASE. Article 30 of the Original Lease is hereby
         amended to provide that at the request of either party, the notice of
         lease relating to the Original Lease shall be amended to reflect the
         addition of the Additional Space.

21.      EFFECT. The Lease, as modified by this Third Amendment, remains in full
         force and effect.

22.      MERGER. All understandings, letters of intent or agreements between
         Tenant and Landlord, which predate this Third Amendment are merged
         herein. No oral statements or representations or prior written
         communications by or between the parties dealing with the subject
         matter of this Third Amendment shall be binding or effective. This
         Third Amendment and the Lease are the sole and complete expression of
         the agreement between Landlord and Tenant as to the subject matter
         thereof.

23.      SEVERABILITY. If any of the provisions of this Third Amendment, or its
         application, is held by any court or in arbitration to be invalid or
         inapplicable, such decision shall not affect any other term, provision,
         covenant or condition of this Third Amendment. Notwithstanding the
         foregoing, if the invalid provision has the effect of reducing the rent
         to be paid by Tenant, Landlord may cancel the Lease.

24.      GOVERNING LAW. The laws of the State of Connecticut will govern the
         interpretation of this Third Amendment.

25.      BINDING NATURE AND TIME IS OF THE ESSENCE. This Third Amendment shall
         be binding upon the parties hereto and upon their heirs,
         administrators, executors, successors and assigns, and shall not be
         construed against the party that drafted it. The paragraph headings are
         for the parties' convenience and shall not be deemed to effect the
         meaning of this Third Amendment or otherwise. Time is of the essence
         with respect to all dates and


                                       13
<PAGE>

         time periods set forth in this Third Amendment.




                                       14
<PAGE>

         IN WITNESS WHEREOF, the parties hereto have hereunto set their hands
and seals as of the day and year first above written.

SCIENCE PARK DEVELOPMENT CORPORATION


By:      /s/ Dennis C. Lyndon
         ------------------------------
         Dennis C. Lyndon
         Director of Development
         Duly Authorized



GENAISSANCE PHARMACEUTICALS, INC.


By:      /s/ Kevin Rakin
         ------------------------------
         Kevin Rakin
         Executive Vice President
         Duly Authorized


                                       15
<PAGE>

         Pursuant to Section 34.2 of the Lease dated September 15, 1998 by and
between Science Park Development Corporation and Genaissance Pharmaceuticals,
Inc., the undersigned hereby consent to the foregoing Third Amendment to Lease.

CONNECTICUT DEVELOPMENT AUTHORITY


By:      Douglas G. Twele                   Date:    7/26/00
        -----------------------------              ------------------------
         Print Name:
         Print Title:

THE CONNECTICUT HOUSING FINANCE AUTHORITY


By:  /s/ John K. Claford                    Date:    7/24/00
        -----------------------------              ------------------------
         Print Name:
         Print Title:

CONNECTICUT INNOVATIONS, INC.


By:  /s/ Victor R. Budnick                  Date:    7/25/00
        -----------------------------              ------------------------
         Print Name:
         Print Title:


                                       16
<PAGE>

                                   EXHIBIT A-1

                     FLOOR PLAN OF 1ST FLOOR EAST WING SPACE
                        AND PROPOSED FIRST FLOOR WALKWAY


                                       1
<PAGE>

                                   EXHIBIT A-2

                             FLOOR PLAN OF PENTHOUSE



                                       1
<PAGE>

                                   EXHIBIT A-3

                                THIRD FLOOR SPACE



                                       1
<PAGE>

                                   EXHIBIT A-4

                            DESIGNATED PARKING AREAS



                                       2
<PAGE>

                                   EXHIBIT B-1

                                  INTERIOR WORK


1.       Piping and supply of chilled and hot water sufficient to operate
         Tenant's HVAC system will be installed up to the Additional Space.

2.       Modifications and/or upgrades to fire alarm system and sprinkler system
         if required by fire marshal.

3.       Installation of hot water supply and return.

4.       Installation of baseboard heating system.

5.       Window replacement work.


                                       3