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                                                                    Exhibit 10.7


                            FIRST AMENDMENT TO LEASE
                            ------------------------

     This First Amendment to Lease dated this 2nd day of December, 1996 between
CambridgePark One Limited Partnership ("Landlord") and Gensym Corporation
("Tenant").

                                   WITNESSETH

     WHEREAS, Landlord and Tenant entered into a lease dated as of January 1,
1995 for space located in the building known as 125 CambridgePark Drive,
Cambridge, Massachusetts (the "Lease"); and

     WHEREAS, Landlord and Tenant wish to amend the lease as set forth herein
for purposes of expanding the premises which Tenant presently occupies.

     NOW, therefore, for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, Landlord and Tenant agree to amend
the lease as follows:

     1. All capitalized terms used herein and not defined herein shall have the
meanings ascribed to them in the lease.

     2. Landlord and Tenant acknowledge and agree that, in accordance with the
provisions of Section 2.1 of the Lease, the original Premises (consisting of the
First Takedown Space) was expanded effective on October 1, 1995 to include the
Third TakeDown Space (the "Effective Date"). Notwithstanding the provisions of
the Lease, the parties acknowledge that the Third Takedown Space consists of
3,642 rentable square feet as shown on page A-3 of Exhibit A attached hereto.
The Lease is hereby amended to delete page A-3 of Exhibit A to the Lease headed
"Third Takedown Space" and to substitute the attached page A-3 of Exhibit A
therefor. Accordingly, as of the Effective Date, the Premises consisted of
47,022 rentable square feet.

     3. Section 1.1 of the lease is hereby amended by inserting the following on
page 2 after the data following the subject "Fourth Takedown Space":

     FIFTH TAKEDOWN SPACE:  Approximately 4,879 r.s.f., located on the third
                            floor of the Building, being the cross-hatched area
                            shown on EXHIBIT A, which shall be automatically
                            included in the




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                            Premises on the Substantial Completion Date for the
                            Fifth Takedown Space, in accordance with
                            Section 2.1 hereof.

     SIXTH TAKEDOWN SPACE:  Approximately 3,333 r.s.f., located on the third
                            floor of the Building, being the cross-hatched area
                            shown on EXHIBIT A, which shall be automatically
                            included in the Premises on the Substantial
                            Completion Date for the Sixth Takedown Space, in
                            accordance with Section 2.1 hereof.

     ORIGINAL TAKEDOWN
     SPACES:                The First Takedown Space, the Second Takedown Space,
                            the Third Takedown Space and the Fourth Takedown
                            Space.

     NEW TAKEDOWN SPACES:   The Fifth Takedown Space and the Sixth Takedown
                            Space.

     4. Section 1.1 of the Lease is hereby amended by deleting the data which
corresponds to the subject "Annual Base Rent" and replacing it with the
following data:

        Lease Year                    Annual Base Rent (p.r.s.f.)
        ----------                    ----------------

            1               $7.00 for the Original Takedown Spaces

            2               $7.25 for the Original Takedown Spaces and,
                            effective on expansion of the Premises to include
                            New Takedown Space as provided herein, $11.50 for
                            the New Takedown Spaces

            3               $7.50 for the Original Takedown Spaces and $11.50
                            for the New Takedown Spaces

            4               $7.75 for the Original Takedown Spaces and $11.50
                            for the New Takedown Spaces


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            5               $8.00 for the Original Takedown Spaces and $11.50
                            for the New Takedown Spaces

            6               $8.25 for the Original Takedown Spaces and $11.50
                            for the New Takedown Spaces

     5. Section 1.1 of the Lease is hereby amended by deleting the data which
corresponds to the subject "Annual Rent" and replacing it with the following
data:


        Lease Year                      Annual Rent
        ----------                      -----------

            1               $7.00 Annual Base Rent (p.r.s.f.) + $9.50 Annual
                            Estimated Operating Costs (p.r.s.f.) + $.85 Annual
                            Estimated Electrical Costs to Tenant's Space
                            (p.r.s.f.) = $17.35 (p.r.s.f.) x Rentable Floor Area
                            of the Original Expansion Spaces as the same may be
                            increased during the Lease Year in accordance with
                            Section 2.1 and Section 3.2 = Annual Rent.

            2               The sum of: (a) with respect to the Original
                            Expansion Spaces, $7.25 Annual Base Rent (p.r.s.f.)
                            + $9.50 Annual Estimated Operating costs (p.r.s.f.)
                            + $.85 Annual Estimated Electrical Costs to Tenant's
                            Space (p.r.s.f.) = $17.60 (p.r.s.f.) x Rentable
                            Floor Area of the Original Expansion Spaces, as the
                            same may be increased during the Lease Year in
                            accordance with Section 2.1 and Section 3.2 = Annual
                            Rent for the Original Expansion Spaces, and (b) with
                            respect to the New Expansion Spaces, $11.50 Annual
                            Base Rent (p.r.s.f.) + $9.50 Annual Estimated
                            Operating Costs (p.r.s.f.) + $.85 Annual Estimated
                            Electrical Costs to Tenant's Space (p.r.s.f.) =
                            $21.85 (p.r.s.f.) x Rentable Floor Area of the New
                            Expansion Spaces as the same



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                            may be increased during the Lease Year in accordance
                            with Section 2.1 and Section 3.2 = Annual Rent for
                            the New Expansion Spaces.

            3               The sum of: (a) with respect to the Original
                            Expansion Spaces, $7.50 Annual Base Rent (p.r.s.f.)
                            + $9.50 Annual Estimated Operating Costs (p.r.s.f.)
                            + $.85 Annual Estimated Electrical Costs to Tenant's
                            Space (p.r.s.f.) = $17.85 (p.r.s.f.) x Rentable
                            Floor Area of the Original Expansion Spaces, as the
                            same may be increased during the Lease Year in
                            accordance with Section 2.1 and Section 3.2 = Annual
                            Rent for the Original Expansion Spaces, and (b) with
                            respect to the New Expansion Spaces, $11.50 Annual
                            Base Rent (p.r.s.f.) + $9.50 Annual Estimated
                            Operating Costs (p.r.s.f.) + $.85 Annual Estimated
                            Electrical Costs to Tenant's Space (p.r.s.f.) =
                            $21.85 (p.r.s.f.) x Rentable Floor Area of the New
                            Expansion Spaces as the same may be increased during
                            the Lease Year in accordance with Section 2.1 and
                            Section 3.2 = Annual Rent for the New Expansion
                            Spaces.

            4               The sum of: (a) with respect to the Original
                            Expansion Spaces, $7.75 Annual Base Rent (p.r.s.f.)
                            + $9.50 Annual Estimated Operating costs (p.r.s.f.)
                            + $.85 Annual Estimated Electrical Costs to Tenant's
                            Space (p.r.s.f.) = $18.10 (p.r.s.f.) x Rentable
                            Floor Area of the Original Expansion Spaces, as the
                            same may be increased during the Lease Year in
                            accordance with Section 2.1 and Section 3.2 = Annual
                            Rent for the Original Expansion Spaces, and (b) with
                            respect to the New Expansion Spaces, $11.50 Annual
                            Base Rent (p.r.s.f.) + $9.50 Annual Estimated
                            Operating Costs (p.r.s.f.) + $.85 Annual Estimated
                            Electrical Costs to Tenant's Space (p.r.s.f.) =
                            $21.85

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                            (p.r.s.f.) x Rentable Floor Area of the New
                            Expansion Spaces as the same may be increased during
                            the Lease Year in accordance with Section 2.1 and
                            Section 3.2 = Annual Rent for the New Expansion
                            Spaces.

            5               The sum of: (a) with respect to the Original
                            Expansion Spaces, $8.00 Annual Base Rent (p.r.s.f.)
                            + $9.50 Annual Estimated Operating costs (p.r.s.f.)
                            + $.85 Annual Estimated Electrical Costs to Tenant's
                            Space (p.r.s.f.) = $18.35 (p.r.s.f.) x Rentable
                            Floor Area of the Original Expansion Spaces, as the
                            same may be increased during the Lease Year in
                            accordance with Section 2.1 and Section 3.2 = Annual
                            Rent for the Original Expansion Spaces, and (b) with
                            respect to the New Expansion Spaces, $11.50 Annual
                            Base Rent (p.r.s.f.) + $9.50 Annual Estimated
                            Operating Costs (p.r.s.f.) + $.85 Annual Estimated
                            Electrical Costs to Tenant's Space (p.r.s.f.) =
                            $21.85 (p.r.s.f.) x Rentable Floor Area of the New
                            Expansion Spaces as the same may be increased during
                            the Lease Year in accordance with Section 2.1 and
                            Section 3.2 = Annual Rent for the New Expansion
                            Spaces.

            6               The sum of: (a) with respect to the Original
                            Expansion Spaces, $8.25 Annual Base Rent (p.r.s.f.)
                            + $9.50 Annual Estimated Operating costs (p.r.s.f.)
                            + $.85 Annual Estimated Electrical Costs to Tenant's
                            Space (p.r.s.f.) = $18.60 (p.r.s.f.) x Rentable
                            Floor Area of the Original Expansion Spaces, as the
                            same may be increased during the Lease Year in
                            accordance with Section 2.1 and Section 3.2 = Annual
                            Rent for the Original Expansion Spaces, and (b) with
                            respect to the New Expansion Spaces, $11.50 Annual
                            Base Rent (p.r.s.f.) + $9.50 Annual Estimated
                            Operating Costs (p.r.s.f.) + $.85


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                            Annual Estimated Electrical Costs to Tenant's Space
                            (p.r.s.f.) = $21.85 (p.r.s.f.) x Rentable Floor Area
                            of the New Expansion Spaces as the same may be
                            increased during the Lease Year in accordance with
                            Section 2.1 and Section 3.2 = Annual Rent for the
                            New Expansion Spaces

     6. Section 1.1 of the Lease is hereby amended by inserting the word
"Original" before "Takedown Space" in the data following the subject "Tenant's
Design Completion Date"

     7. Section 1.1 of the Existing Lease is hereby amended by inserting the
following after the data following the subject "Scheduled Substantial Completion
Dates":

Fifth Takedown Space:       October 15, 1996
Sixth Takedown Space:       45 days after the Sixth Takedown Space Possession
                            Date (as defined in Section 3.1(a) hereof)

     8. Section 3.1 of the Lease is hereby amended by inserting the word
"Original" before the phrase "Takedown Space" in the second and seventh lines of
subsection (a) and wherever such phrase appears in subsection (b), except where
reference is made to the First Takedown Space, Second Takedown Space, Third
Takedown Space or the Fourth Takedown Space, and by inserting the following at
the end of Section 3.1:

          "Notwithstanding any other provision of the Lease to the contrary, the
     Fifth Takedown Space and the Sixth Takedown Space shall be provided by
     Landlord to Tenant, and accepted by Tenant from Landlord, "as is", "where
     is", with all faults and without any representation, warranty, or guaranty
     of any kind by Landlord to Tenant, except that Landlord shall, at
     Landlord's expense, remove and construct any demising walls which are
     required in connection with the Fifth Takedown Space and the Sixth Takedown
     Space, which construction shall be consistent with Building standards.
     Subject to all provisions hereof other than the payment of Annual Rent,
     Tenant shall be given access to the Fifth Takedown Space to commence
     improvements to the Fifth Takedown Space on October 1, 1996. Subject to all
     provisions hereof other than the payment of Annual Rent, Tenant shall be
     given access to the Sixth Takedown Space to commence improvements to the
     Sixth Takedown Space no later than five (5) business days after the Tenant
     currently located in such space is relocated by Landlord or otherwise quits
     the Sixth Takedown Space (such

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     date on which Tenant is given access being referred to herein as the "Sixth
     Takedown Space Possession Date"). Within thirty (30) days after notice from
     Tenant to Landlord that Tenant's construction of improvements in the Fifth
     Takedown Space or the Sixth Takedown Space, as applicable, has been
     completed, together with an itemization of and paid invoices for the
     out-of-pocket costs incurred by Tenant in constructing such improvements,
     Landlord shall pay to Tenant an allowance equal to the lesser of such costs
     or $7.50 (plus architectural and engineering fees) per Rentable Square Foot
     of such space. All construction to be undertaken by Tenant shall be subject
     to the applicable provisions of the Lease, including, without limitation,
     applicable provisions of Sections 3.1, 3.2 and 6.1.15."


     9. Section 3.2 of the Lease is hereby amended by inserting the word
"Original" before the phrase "Takedown Space" wherever such phrase appears in
Section 3.2, and by inserting the following at the end of Section 3.2:

          "Tenant agrees to use reasonable efforts to have each New Takedown
     Space ready for occupancy on or before the applicable Scheduled Substantial
     Completion Date for each such New Takedown Space, which shall, however, be
     extended in the case of each New Takedown Space for a period equal to that
     of any delays due to governmental regulations, unusual scarcity of or
     inability to obtain labor or materials, labor difficulties, casualty or
     other causes beyond Tenant's reasonable control. As used herein, the term
     "Substantial Completion Date" for each New Takedown Space shall mean and
     refer to the date on which the improvements to such New Takedown Space have
     been substantially completed as reasonably determined by Landlord's
     representative, with the exception of minor items which can be fully
     completed without material interference with Tenant, provided that none of
     said items are necessary to make the applicable New Takedown Space
     tenantable for the permitted Uses; provided, however, that if completion is
     delayed due to Tenant's failure to proceed with construction diligently and
     in good faith or otherwise due to delays caused by Tenant, then the
     applicable New Takedown Space shall be deemed ready for occupancy no later
     than the Scheduled Substantial Completion Date applicable to such New
     Takedown Space, upon which date the Annual Rent, the Rentable Floor Area of
     Tenant's Space and all other charges due hereunder shall be adjusted in
     accordance with Sections 2.1, 2.3, and 3.2 as though the Substantial
     Completion Date for such New Takedown Space had actually occurred on such
     Scheduled Substantial Completion Date."

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     10. Pages A-5 and A-6 attached hereto are hereby attached to the Lease as
pages A-5 and A-6 of EXHIBIT A to the Lease.

     11. Exhibit G to the Lease is hereby amended by revising the chart for the
"Number of Cars to be Issued for:" with respect to the Third Takedown Space by
substituting the number "3" for the number "5" in the column headed "Blue Lot"
and the number "8" for the number "10" in the column headed "Red Lot" and by
adding the following information in such chart in the corresponding columns:

         Upon Occupancy of %         Blue Lot       Red Lot
         -------------------         --------       -------

         Fifth Takedown Space          5              10
         Sixth Takedown Space          3               7

     12. Tenant and Landlord represent and warrant to each other that they have
dealt with no brokers in connection with this First Amendment to Lease other
than Spaulding and Slye Collier's Limited Partnership and agree to defend, with
counsel approved by the other, indemnify and save the other harmless from and
against any and all cost, expense or liability for any compensation, commissions
or charges claimed by a broker or agent, other than Spaulding and Slye Collier's
Limited Partnership in connection with this First Amendment to Lease.

     13. Except as set forth herein, the Lease shall remain unmodified and in
full force and effect.

     EXECUTED as a sealed instrument as of the day and year first written above.


                                        LANDLORD

                                        CAMBRIDGEPARK ONE LIMITED
                                        PARTNERSHIP, by its Agent,
                                        Spaulding and Slye Services Limited
                                        Partnership

                                             By: /s/ Robert Lemons
                                                --------------------------------
                                                Name: Robert Lemons
                                                Title: Senior Vice President

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                                        TENANT

                                        GENSYM CORPORATION

                                        By: /s/ Stephen Gregorio
                                           -------------------------------------
                                           Name: Stephen Gregorio
                                           Title: Vice President of Finance
                                                   and Administration and C.F.O.

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