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                                                                  EXHIBIT 10.13


                            FOURTH AMENDMENT TO LEASE



LESSOR:           OLD KENDALL PROPERTY LLC

LESSEE:           CAMBRIDGE NEUROSCIENCE INC.

DATE OF LEASE:    JULY 16, 1992;
                  FIRST AMENDMENT DATED SEPTEMBER 22, 1992;
                  SECOND AMENDMENT DATED SEPTEMBER 22, 1993;
                  THIRD AMENDMENT DATED MARCH 11, 1996.

PREMISES:         ONE KENDALL SQUARE - BLDG. 650 AND BLDG. 700.



In consideration of Ten ($10.00) Dollars and other valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, the above-described
lease is hereby amended effective June 1, 1997 as follows:


1.   TERM.

     The term of the lease shall be extended through July 31, 2000.


2.   RENT.

     Effective January 1, 1998, the Base Rent on the Leased Premises shall be
adjusted as follows:

     Commencing on July 1, 1998, LESSEE agrees to pay Base Rent on the Leased
Premises as follows:

          (i) $1,109,790.000 per annum ($28.50 per RSF times 38,940 RSF), (or
          $92,482.50 monthly); plus

          (ii) The annual fair rental value as reasonably determined by LESSOR
          of LESSEE's parking spaces used in the OKS Garage ("Garage Parking").

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                                                                  EXHIBIT 10.13

3.   COMPLETION OF IMPROVEMENTS.

     The Leased Premises shall be delivered to LESSEE in their current "As is"
condition.


4.   BROKER.

     The LESSOR and LESSEE each represent and warrant to the other that each has
had no dealings with any Brokers concerning this Third Amendment to Lease,
except ROBERT A. JONES & COMPANY and FALLON, HINES & O'CONNOR and each party
agrees to indemnify and hold the other harmless for any damages occasioned to
the other by reason of a breach of this representation and warranty. LESSOR
agrees to pay any commission due to LESSEE's broker.


5.   OPTION TO EXPAND.

     On or before December 31, 1997, LESSOR shall notify LESSEE whether or not
the Digital Equipment Corporation space ("Digital Space") containing 31,747
rentable square feet on the second floor of Buildings 600/650/700 will become
available on May 1, 1998. If LESSOR notifies LESSEE that the space will be
available for lease and if LESSEE is not then in default under this lease beyond
applicable notice, grace and cure periods, LESSEE shall have the option upon
written notice to LESSOR within ten (10) days of the above availability notice
to notify LESSOR that it will occupy the Digital Space on the terms and
conditions provided below. If LESSEE does not exercise its option to take the
Digital Space, all rights to said space shall cease. If LESSEE does elect to
occupy the Digital Space, the initial term shall be coterminous with this fourth
amendment through July 31, 2000. The Digital Space shall be delivered in its "as
is" condition. Thirty (30) days after delivery of the Digital Space to LESSEE,
LESSEE shall vacate and cease being obligated for any rent on all of its current
first floor space (11,642 RSF) in Bldg. 700. At that same time, Base Rent
pursuant to paragraph 2(i) above shall be changed to: $1,515,451.50 per annum
(representing $34.00 per RSF times 27,298 RSF plus $18.50 per RSF times 31,747
RSF), or $126,287.63 monthly. LESSEE shall be responsible for its pro rata share
of CAO costs and Real Estate Taxes for the Lot and Building on the Digital
Space. Upon occupancy of the Digital Space and the return of the first floor
Bldg. 700 space, LESSEE's (on 59,045 RSF) proportionate Lot share shall be 9.67%
and its Proportionate Building share shall be 26.19%.


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                                                                  EXHIBIT 10.13

     6.   OPTION TO EXTEND LEASE FIVE (5) YEARS FROM AUGUST 1, 2000.

     If the LESSEE is not then in default beyond applicable notice, grace and
cure periods, LESSOR does hereby grant to LESSEE the option to extend this Lease
for one (1) additional five- (5) year term, commencing on the expiration of the
term of this amendment (July 31, 2000) upon the same terms and conditions as
herein contained except the annual Base Rent set forth in Paragraph 4 hereof
shall be at the rate set forth below. The option shall be exercised by written
notice from LESSEE and received by LESSOR on or before July 31, 1999. The annual
rent for the Extended Term shall be adjusted at the commencement of the Extended
Term and shall be as set forth below.

     In the event the LESSEE has decided not to occupy the Digital Space
pursuant to Paragraph 5 above, the Base Rent shall be $1,002,705 per annum
(representing $25.75 per RSF times 38,940 RSF), or $83,558.75 monthly. The space
will be delivered in its "as is" condition.

     In the event the LESSEE has elected to occupy the Digital Space pursuant to
Paragraph 5 above, the Base Rent for Years 1 and 2 shall be $1,399,307.50 per
annum (representing $23.50 per RSF times 59,545 RSF) and for Years 3 through 5
shall be $1,458,852.50 per annum (representing $24.50 per RSF times 59,545 RSF).
LESSOR shall provide a $317,470 tenant improvement allowance to LESSEE for work
on the Digital Space, all work subject to LESSOR's reasonable approval.

     7.   ADDITIONAL OPTIONS TO EXTEND.

     If the LESSEE is not then in default beyond applicable notice, grace and
cure periods, and if LESSEE has previously exercised its option under Paragraph
6 above, LESSOR does hereby grant to LESSEE the option to extend this Lease for
two (2) additional five- (5) year terms, commencing on the expiration of the
Option Term per Paragraph 6 above upon the same terms and conditions as herein
contained except the annual Base Rent set forth in Paragraph 4 hereof shall be
at the rate set forth below. The option shall be exercised by written notice
from LESSEE and received by LESSOR at least eight (8) months prior to the
expiration of the prior term. The annual rent for the Extended Terms shall be
adjusted at the commencement of each Extended Term and shall be at fair market
value as determined below.

     Fair market value shall mean the then prevailing rents and charges for
comparable rental properties in Cambridge, Massachusetts, taking into account
the then condition of the Leased Premises. Within 30 days after LESSEE exercises
its option to extend, LESSOR shall notify LESSEE in writing of LESSOR'S
determination of the market rate ("LESSOR'S Rental Notice"). Upon LESSEE'S
receipt of LESSOR's Rental Notice, LESSEE may elect to nullify its exercise of
the option to extend by giving LESSOR written notice of such nullification
within ten (10) days after receipt of LESSOR'S Rental Notice; upon the giving of
such nullification notice,

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                                                                  EXHIBIT 10.13

this lease shall automatically terminate as of the originally scheduled
expiration date of the initial Term and LESSEE shall have no right to extend the
term. If LESSEE has not so nullified its exercise of the extension option and
within thirty (30) days after the LESSEE has receive the LESSOR'S Rental Notice,
the parties cannot agree upon the market rate, then the market rate shall be
submitted to arbitration as follows: market rate shall be determined by
impartial arbitrators (who shall be qualified real estate appraisers or brokers
dealing with like types of properties), one to be chosen by the LESSOR, one to
be chosen by the LESSEE, and a third to be selected, if necessary as below
provided. The unanimous written decision of the two first chosen (without
selection and participation of the third arbitrator), or otherwise the written
decision of a majority of the three arbitrators chosen and selected as
aforesaid, shall be conclusive and binding upon LESSOR and LESSEE. LESSOR and
LESSEE shall each notify the other of its chosen arbitrator within ten (10) days
following the call for arbitration and, unless such arbitrators shall have
reached an unanimous decision within thirty (30) days after their designation,
they shall so notify the then President of the Greater Boston Real Estate Board
and request him to select an impartial third arbitrator to determine the market
rate as herein defined. Such third arbitrator and the first two chosen shall
render their decision within thirty (30) days following the date of appointment
of the third arbitrator and shall notify LESSOR and LESSEE thereof. LESSOR and
LESSEE shall share (50/50) all expenses of arbitration, regardless of the
outcome of arbitration. If the dispute between the parties as to a market rate
has not been resolved before the commencement date of LESSEE'S obligations to
pay Annual Rent based upon such market rate, LESSEE shall pay Annual Rent under
the Lease based upon the market rate designated by a LESSOR until either (i)
agreement of the parties as to the market rate, or (ii) decision of the
arbitrators, as the case may be, at which time LESSEE shall promptly pay any
underpayment of Annual Rent to LESSOR , or LESSOR shall credit the overpayment
of Annual Rent against the next installment of rental or other charges due to
LESSOR hereunder.

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                                                                 EXHIBIT 10.13

     Except as herein expressly amended, the above-captioned Lease shall remain
unaltered, in full force and effect, and is hereby reaffirmed.


     WITNESS our hands and seals this 17th day of June 1997.


LESSOR:
OLD KENDALL PROPERTY LLC



BY  /S/ ALLAN R. JONES                                /S/ TINA BORGES-DRUTH
  ----------------------------------------            ------------------------
    NAME:    ALLAN R. JONES                           WITNESS
    OFFICE:  PRESIDENT OF KENDALL F.A. INC.,
             MANAGING MEMBER
             HEREUNTO DULY AUTHORIZED




LESSEE
CAMBRIDGE NEUROSCIENCE, INC.



BY  /S/ ELKAN R. GAMZU                                /S/ ROSS S. GIBSON
  ----------------------------------------            ------------------------
    NAME:                                             WITNESS
    OFFICE:  PRESIDENT
             HEREUNTO DULY AUTHORIZED




BY  /S/ HARRY W. WILCOX                               /S/ ROSS S. GIBSON
  ----------------------------------------            ------------------------
    NAME:                                             WITNESS
    OFFICE: TREASURER
            HEREUNTO DULY AUTHORIZED




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