Document:

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                                                                   Exhibit 10.7

                              WAIVER AND AMENDMENT

         This Waiver and Amendment, dated as of January 26, 2001 (this "Waiver
and Amendment"), waives and amends certain provisions of the Stock and Warrant
Purchase Agreement, dated as of May 13, 1997 (the "Stock and Warrant Purchase
Agreement"), between SYLAMERICA, INC. (now Sanofi-Synthelabo Inc.)
("Sanofi-Synthelabo") and PRAECIS PHARMACEUTICALS INCORPORATED (the "Company").
Capitalized terms used herein and not otherwise defined shall have the meanings
ascribed to them in the Stock and Warrant Purchase Agreement. All references
herein to sections shall be understood to be references to sections of the Stock
and Warrant Purchase Agreement.

         WHEREAS, on January 18, 2001, the Company, in accordance with Section
6.1 of the Stock and Warrant Purchase Agreement, provided notice to
Sanofi-Synthelabo of a proposed firm commitment underwritten public offering of
shares of its common stock for which a Registration Statement on Form S-1 was
filed with the Securities and Exchange Commission on January 26, 2001 (the
"Offering");

         WHEREAS, the Company has requested that Sanofi-Synthelabo waive its
right to include Registrable Securities in the Offering pursuant to Section 6.1
of the Stock and Warrant Purchase Agreement, in consideration for the Company
agreeing to amend the Stock and Warrant Purchase Agreement as provided herein;

<PAGE>

         WHEREAS, Sanofi-Synthelabo has agreed to waive its right to include
Registrable Securities in the Offering; and

         WHEREAS, in consideration of Sanofi-Synthelabo's waiver of such right,
the Company has agreed to amend Section 6.1(c) of the Stock and Warrant Purchase
Agreement as provided herein.

         NOW, THEREFORE, the parties hereto hereby agree as follows:

         1. WAIVER. Sanofi-Synthelabo hereby irrevocably waives any right it may
otherwise have pursuant to Section 6.1 of the Stock and Warrant Purchase
Agreement to include Registrable Securities in the Offering.

         2. AMENDMENT. Section 6.2(c) of the Stock and Warrant Purchase
Agreement is hereby amended in its entirety to read as follows:

         (c) If a registration undertaken by PRAECIS involves an Underwritten
         Offering, Purchaser, if and only if any Registrable Securities are
         included in such registration, will, if requested by the Managing
         Underwriter, enter into an agreement not to effect any public sale or
         distribution, including any sale pursuant to Rule 144 under the
         Securities Act, of any Registrable Securities, or of any security
         convertible into or exchangeable or exercisable for any Registrable
         Securities (other than as part of such Underwritten Offering), without
         the consent of the Managing Underwriter, during a period commencing on
         the effective date of such registration and ending a number of calendar
         days thereafter not exceeding 180 as the Board of Directors of PRAECIS
         and the Managing Underwriter shall reasonably determine is required to
         effect a successful offering.

         3. APPLICABILITY OF AMENDMENT TO THE OFFERING. For the avoidance of
doubt, the parties acknowledge that the amendment of Section 6.2(c) of the Stock

                                       2

<PAGE>

and Warrant Purchase Agreement effected by Section 2 hereof shall apply with
respect to the Offering and, accordingly, Purchaser shall have no obligation to
enter into an agreement referred to in Section 6.2(c), as amended, in connection
with the Offering.

         4. PARTIES IN INTEREST. This Waiver and Amendment shall be binding upon
and inure solely to the benefit of each party hereto and its respective
successors and assigns. Nothing in this Waiver and Amendment, express or
implied, is intended to or shall confer upon any other person any rights,
benefits or remedies of any nature whatsoever under or by reason of this Waiver
and Amendment.

         5. COUNTERPARTS. This Waiver and Amendment may be executed in
counterparts, each of which shall be deemed an original but which together shall
constitute one and the same instrument.

         6. GOVERNING LAW. This Waiver and Amendment shall be governed by and
construed in accordance with the laws of the State of Delaware, without giving
effect to the conflicts of law principles thereof.

            [The remainder of this page is intentionally left blank.]

                                       3
<PAGE>

         IN WITNESS WHEREOF, each of the parties has executed this Waiver and
Amendment as of the date first above written.

                               PRAECIS PHARMACEUTICALS INCORPORATED

                               By /s/ Kevin F. McLaughlin
                                  -----------------------------
                               Name: Kevin F. McLaughlin
                               Title: Sr. V.P. and CFO

                               SANOFI-SYNTHELABO INC.

                               By /s/ Gregory Irace
                                  -----------------------------
                               Name: Gregory Irace
                               Title: Vice President and Chief Financial Officer

                               By /s/ Richard H. Thomson
                                  -----------------------------
                               Name: Richard H. Thomson
                               Title: Vice President and Treasurer

                                      4<PAGE>
                                                                   Exhibit 10.8

                [PRAECIS PHARMACEUTICALS INCORPORATED LETTERHEAD]

                                                              January 30, 2001

VIA FACSIMILE AND FEDEX
-----------------------

Sanofi-Synthelabo Inc.
90 Park Avenue
New York, NY 10016
Facsimile No. 212-551-4921
Attention: John M. Spinnato,
           Senior Vice President and General Counsel

                  Re:   Amendment to Warrant for the Purchase of Shares of
                        Common Stock of PRAECIS PHARMACEUTICALS
                        INCORPORATED
                        ------------

Dear Sirs:

                  This letter will constitute an amendment, effective as of the
date hereof, to the Warrant for the Purchase of Shares of Common Stock (the
"Warrant") originally issued by PRAECIS PHARMACEUTICALS INCORPORATED (the
"Company") to Sylamerica, Inc., now known as Sanofi-Synthelabo Inc.
("Sanofi-Synthelabo"), pursuant to the Stock and Warrant Purchase Agreement,
dated as of May 13, 1997, between Sylamerica, Inc. and the Company. Capitalized
terms used herein and not otherwise defined shall have the meanings ascribed to
them in the Warrant.

         The parties hereby agree that the Warrant is amended as follows:

         -  Section 1. of the existing Warrant is renumbered as "Section 1.(a)".

         -  A new paragraph (b) to Section 1. is added to the Warrant and reads
            in its entirety as follows:

            1.(b)   Notwithstanding paragraph (a) of this Section 1., in lieu of
            payment of the Exercise Price, the Holder may exercise this Warrant,
            in whole or in part, by surrender of this Warrant, together with a
            Cashless Exercise Form in the form attached hereto as Exhibit II,
            duly executed

<PAGE>

Sanofi-Synthelabo Inc.
Page 2

            (a "Cashless Exercise"). Acceptance by the Company of such
            presentation and surrender shall be deemed a waiver of the Holder's
            obligation to pay all or any part of the Exercise Price for the
            Shares covered by such exercise. In the event of a Cashless
            Exercise, the Holder shall exchange this Warrant for that number of
            shares of Common Stock (rounded up to the nearest whole share,
            notwithstanding the provisions of Section 2 of this Warrant relating
            to the treatment of fractional shares) determined by multiplying the
            number of Shares for which the Warrant is being so exercised by a
            fraction, the numerator of which shall be the positive difference
            (if any) between the then current market price per share of the
            Common Stock and the Exercise Price, and the denominator of which
            shall be the then current market price per share of Common Stock.
            For purposes of any computation under this Section 1(b), the then
            current market price per share of Common Stock shall be deemed to be
            the closing sales price per share of the Common Stock on the
            principal public trading market on which the Common Stock is then
            traded on the business day immediately preceding the date of the
            Cashless Exercise, or if the Common Stock is not then traded on any
            public trading market, the fair market price of the Common Stock as
            at such date, as determined in good faith by the Company's Board of
            Directors.

            - A new Exhibit II is added to the Warrant and reads in its
              entirety as provided in the attached form of Exhibit II.

            All other terms and provisions of the Warrant will remain in full
force and effect.

            The Company and Sanofi-Synthelabo acknowledge, by their respective
signatures below, that no commission, remuneration or other consideration was
paid or given by them in connection with the matters set forth herein.

<PAGE>

Sanofi-Synthelabo Inc.
Page 3

            Please acknowledge your acceptance of and agreement with the
foregoing by signing below in the space provided and returning a copy of this
letter to Kevin F. McLaughlin at the address set forth above.

                                                Very truly yours,

                                                PRAECIS PHARMACEUTICALS
                                                INCORPORATED

                                                By /s/ Kevin F. McLaughlin
                                                   ----------------------------
                                                   Name: Kevin F. McLaughlin
                                                   Title:  Sr. V.P. and CFO

Enclosure

cc:      Coudert Brothers
         1114 Avenue of the Americas
         New York, NY  10036-7703
         Attention: James Colihan, Esq.

Accepted and Agreed:

SANOFI-SYNTHELABO INC.

By /s/ Gregory Irace
   -----------------------------------------
    Name:  Gregory Irace
    Title: Vice President and CFO

By /s/ Richard H. Thomson
   -----------------------------------------
    Name:  Richard H. Thomson
    Title: Vice President and Treasurer

<PAGE>

                               FORM OF EXHIBIT II

                                                                     Exhibit II
                                                                     ----------

                          CASHLESS ELECTION TO EXERCISE

    The undersigned hereby irrevocably exercises its right to subscribe for and
purchase from PRAECIS PHARMACEUTICALS INCORPORATED, a Delaware corporation,
__________ fully paid, validly issued and nonassessable shares of Common Stock
covered by the within Warrant. The undersigned hereby elects to pay the Exercise
Price pursuant to the Cashless Exercise provisions of the Warrant, as provided
for in Section 1(b) of such Warrant, as amended to date.

                                                    SANOFI-SYNTHELABO INC.

                                                    By________________________
                                                        Name:
                                                        Title:

Date:________________

*********************************************************

INSTRUCTIONS FOR ISSUANCE OF STOCK CERTIFICATE

(if other than to the registered holder of the within Warrant)

Name:____________________________

Address:__________________________
        __________________________
        __________________________
        __________________________

Social Security or
Taxpayer Identification Number:
                               ---------------------------

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