<PAGE>


         THIS WARRANT AND SECURITIES ACQUIRED UPON THE EXERCISE OF THIS WARRANT
         HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED
         (THE "1933 ACT"). THEY MAY NOT BE SOLD, OFFERED FOR SALE, PLEDGED, OR
         HYPOTHECATED IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT
         RELATED THERETO OR AN OPINION OF COUNSEL (WHICH MAY BE COMPANY COUNSEL)
         REASONABLY SATISFACTORY TO THE COMPANY THAT SUCH REGISTRATION IS NOT
         REQUIRED UNDER THE 1933 ACT.

                                WARRANT AGREEMENT

                       To Purchase Shares of Common Stock

                      PRAECIS PHARMACEUTICALS INCORPORATED
                    (formerly Pharmaceutical Peptides, Inc.)

                   Dated as of March 29, 1995 (the "Effective
               Date") Re-Issued as of August 12, 1998 (the "First
                              Re-Issue Date"), and
             again as of July 31, 2000 (the "Second Re-Issue Date")

         WHEREAS, PRAECIS PHARMACEUTICALS INCORPORATED (formerly known as
Pharmaceutical Peptides, Inc.), a Delaware corporation (the "Company"),
entered into a Warrant Agreement dated as of March 29, 1995 (the "Original
Warrant Agreement") with Comdisco, Inc. (the "Original Warrantholder"),
whereby the Company granted the Original Warrantholder the right to purchase
14,925 shares of the Company's Series A Convertible Preferred Stock; and

         WHEREAS, pursuant to and in accordance with the Original Warrant
Agreement, the Original Warrantholder transferred to Gregory Stento (the
"Warrantholder"), effective as of August 12, 1998, the Original
Warrantholder's rights under the Original Warrant Agreement with respect to
the purchase of 1,703 shares of the Company's Series A Convertible Preferred
Stock (the "First Warrant Transfer"), and in connection therewith the Company
entered into a Warrant Agreement with the Warrantholder reflecting and giving
effect to the First Warrant Transfer (the "Original "Stento Warrant"); and

         WHEREAS, pursuant to and in accordance with Section (4)(f) of
Article Fifth of the Company's Amended and Restated Certificate of
Incorporation filed with the Secretary of State of Delaware on April 30, 1998
(hereinafter the "Certificate"), upon the consummation of its initial public
offering on May 2, 2000, the Company converted all of its shares of Preferred
Stock into Common Stock, and in accordance with its terms the Original Stento
Warrant automatically converted into a Warrant to purchase 12,722 shares of
the Company's Common Stock, par value $.01 per share (the "Common Stock").

         WHEREAS, pursuant to and in accordance with the Original Stento
Warrant, the Warrantholder has transferred to Russo and Hale LLP ("Russo and
Hale"), effective as of July 31,




<PAGE>

2000, the Warrantholder's rights under the Original Stento Warrant with
respect to the purchase of 2,862 shares of the Company's Common Stock (the
"Second Warrant Transfer"), and in connection therewith the Company entered
into a Warrant Agreement with Russo and Hale reflecting and giving effect to
the Second Warrant Transfer; and

         WHEREAS, the Company and the Warrantholder acknowledge the Second
Warrant Transfer and, accordingly, the Company is reissuing, as of July 31,
2000, the Warrant provided for in the Original Stento Warrant, and the
Company and the Warrantholder are entering into this Warrant Agreement to
reflect the Second Warrant Transfer and the Warrantholder's right to purchase
9,860 shares of Common Stock as set forth herein; and

         NOW, THEREFORE, in consideration of the foregoing, and the mutual
covenants and agreements contained herein, the Company and the Warrantholder
agree as follows:

1.       GRANT OF THE RIGHT TO PURCHASE COMMON STOCK.

         The Company hereby grants to the Warrantholder, and the
Warrantholder is entitled, upon the terms and subject to the conditions
hereinafter set forth, to subscribe for and purchase from the Company, 9,860
fully paid and non-assessable shares of Common Stock at a purchase price of
$1.35 per share (the "Exercise Price"). The number and purchase price of such
shares are subject to adjustment as provided in Section 8 hereof.

2.       TERM OF THE WARRANT AGREEMENT.

         Except as otherwise provided for herein, the term of this Warrant
Agreement and the right to purchase Common Stock as granted herein commenced
as of the Effective Date and shall be exercisable for a period which
commenced as of the Effective Date and which ends on the later of (i) the
date ten (10) years after the Effective Date, or (ii) the date two (2) years
from the effective date of the Company's initial public offering.

3.       EXERCISE OF THE PURCHASE RIGHTS.

         The purchase rights set forth in this Warrant Agreement are
exercisable by the Warrantholder, in whole or in part, at any time, or from
time to time, prior to the expiration of the term set forth in Section 2
above, by tendering to the Company at its principal office a notice of
exercise in the form attached hereto as Exhibit I (the "Notice of Exercise"),
duly completed and executed. Promptly upon receipt of the Notice of Exercise
and the payment of the purchase price in accordance with the terms set forth
below, and in no event later than twenty-one (21) days thereafter, the
Company shall issue to the Warrantholder a certificate for the number of
shares of Common Stock purchased and shall execute the Acknowledgment of
Exercise in the form attached hereto as Exhibit II indicating the number of
shares which remain subject to future purchases, if any. In the event of an
initial public offering, and if specifically requested by the Underwriter(s),



                                       2


<PAGE>

Warrantholder will agree to lockup provisions equivalent to but not to exceed
those required of the other common shareholders.

         The Exercise Price may be paid at the Warrantholder's election
either (i) by cash or check, or (ii) by surrender of Warrants ("Net
Issuance") as determined below. If the Warrantholder elects the Net Issuance
method, the Company will issue Common Stock in accordance with the following
formula:

                  X = Y(A-B)
                      ------
                        A

Where:            X =      the number of shares of Common Stock to be issued to
                           the Warrantholder.

                  Y =      the number of shares of Common Stock as to which the
                           Warrants are being exercised under this Warrant
                           Agreement.

                  A =      the current fair market value of one (1) share of
                           Common Stock.

                  B =      the Exercise Price.

         As used herein, current fair market value of Common Stock shall mean
with respect to each share of Common Stock:

         (i)      if traded on a securities exchange, the fair market value
         shall be deemed to be the average of the closing prices over a
         twenty-one (21) day period ending three days before the day the current
         fair market value of the securities is being determined; or

         (ii)     if actively traded over-the-counter, the fair market value
         shall be deemed to be the average of the closing bid and asked prices
         quoted on the NASDAQ system (or similar system) over the twenty-one
         (21) day period ending three days before the day the current fair
         market value of the securities is being determined;

         (iii)    if at any time the Common Stock is not listed on any
         securities exchange or quoted in the NASDAQ System or the
         over-the-counter market, the current fair market value of Common Stock
         shall be the highest price per share which the Company could obtain
         from a willing buyer (not a current employee or director) for shares of
         Common Stock sold by the Company, from authorized but unissued shares,
         as determined in good faith by its Board of Directors, unless the
         Company shall become subject to a merger, acquisition or other
         consolidation pursuant to which the Company is not the surviving party,
         in which case the fair market value of Common Stock shall be deemed to
         be the value received by the holders of the Company's Common Stock on a
         common equivalent basis pursuant to such merger or acquisition.



                                       3



<PAGE>

         Upon partial exercise by either cash or Net Issuance, the Company shall
promptly issue an amended Warrant Agreement representing the remaining number of
shares purchasable hereunder. All other terms and conditions of such amended
Warrant Agreement shall be identical to those contained herein, including, but
not limited to the Effective Date hereof.

4.       RESERVATION OF SHARES.

         (a)       AUTHORIZATION AND RESERVATION OF SHARES. During the term
of this Warrant Agreement, the Company will at all times have authorized and
reserved a sufficient number of shares of its Common Stock to provide for the
exercise of the rights to purchase Common Stock as provided for herein.

         (b)       REGISTRATION OR LISTING. If any shares of Common Stock
required to be reserved hereunder require registration with or approval of
any governmental authority under any Federal or State law (other than any
registration under the 1933 Act, as then in effect, or any similar Federal
statute then enforced, or any state securities law), or listing on any
domestic securities exchange, before such shares may be issued, the Company
will, at its expense and as expeditiously as possible, use its reasonable
best efforts to cause such shares to be duly registered, listed or approved
for listing on such domestic securities exchange, as the case may be.

5.       NO FRACTIONAL SHARES OR SCRIP.

         No fractional shares or scrip representing fractional shares shall be
issued upon the exercise of the Warrant, but in lieu of such fractional shares
the Company shall make a cash payment therefor upon the basis of the Exercise
Price then in effect.

6.       NO RIGHTS AS SHAREHOLDER.

         This Warrant Agreement does not entitle the Warrantholder to any voting
rights or other rights as a shareholder of the Company prior to the exercise of
the Warrant.

7.       WARRANTHOLDER REGISTRY.

         The Company shall maintain a registry showing the name and address of
the registered holder of this Warrant Agreement.

8.       ADJUSTMENT RIGHTS.

         The purchase price per share and the number of shares of Common Stock
purchasable hereunder are subject to adjustment, as follows:

         (a)       MERGER AND SALE OF ASSETS. If at any time there shall be a
capital reorganization reclassifying or otherwise altering shares of Common
Stock (other than a combination, reclassifica-



                                       4


<PAGE>

tion, exchange or subdivision of shares otherwise provided for herein), or a
merger or consolidation of the Company with or into another corporation when
the Company is not the surviving corporation, or the sale of all or
substantially all of the Company's properties and assets to any other person
(hereinafter referred to as a "Merger Event"), then, as a part of such Merger
Event, lawful provision shall be made so that the Warrantholder shall
thereafter be entitled to receive, upon exercise of the Warrant, the number
of shares of common stock or other securities of the successor corporation
resulting from such Merger Event, equivalent in value to that which would
have been issuable if Warrantholder had exercised this Warrant immediately
prior to the Merger Event. In any such case, appropriate adjustment (as
determined in good faith by the Company's Board of Directors) shall be made
in the application of the provisions of this Warrant Agreement with respect
to the rights and interest of the Warrantholder after the Merger Event to the
end that the provisions of this Warrant Agreement (including adjustments of
the Exercise Price and number of shares of Common Stock purchasable) shall be
applicable to the greatest extent possible.

         (b)      RECLASSIFICATION OF SHARES. If the Company at any time
shall, by combination, reclassification, exchange or subdivision of
securities or otherwise, change any of the securities as to which purchase
rights under this Warrant Agreement exist into the same or a different number
of securities of any other class or classes, this Warrant Agreement shall
thereafter represent the right to acquire such number and kind of securities
as would have been issuable as the result of such change with respect to the
securities which were subject to the purchase rights under this Warrant
Agreement immediately prior to such combination, reclassification, exchange,
subdivision or other change.

         (c)      SUBDIVISION OR COMBINATION OF SHARES. If the Company at any
time shall combine or subdivide its Common Stock, the Exercise Price shall be
proportionately decreased in the case of a subdivision, or proportionately
increased in the case of a combination.

         (d)      STOCK DIVIDENDS. If the Company at any time shall issue any
shares of Common Stock by means of a dividend or other distribution on the
Common Stock (except any distribution specifically provided for in the
foregoing subsections (a) or (b)), then the Exercise Price shall be adjusted,
from and after the record date of such dividend or distribution, to that
price determined by multiplying the Exercise Price in effect immediately
prior to such record date by a fraction (i) the numerator of which shall be
the total number of shares of Common Stock outstanding immediately prior to
such dividend or distribution, and (ii) the denominator of which shall be the
total number shares of Common Stock outstanding immediately after such
dividend or distribution. The Warrantholder shall thereafter be entitled to
purchase, at the Exercise Price resulting from such adjustment, the number of
shares of Common Stock (calculated to the nearest whole share) obtained by
multiplying the Exercise Price in effect immediately prior to such adjustment
by the number of shares of Common Stock issuable upon the exercise hereof
immediately prior to such adjustment and dividing the product thereof by the
Exercise Price resulting from such adjustment.


                                       5


<PAGE>


         (e)      ANTIDILUTION RIGHTS. A true and correct copy of the
Certificate, as amended through the Second Re-Issue Date, is attached hereto
as Annex A. The Company shall promptly provide the Warrantholder with any
restatement, amendment, modification or waiver of the Certificate.

         (f)      NOTICE OF ADJUSTMENTS. If: (i) the Company shall declare
any dividend or distribution upon its Common Stock, whether in cash,
property, stock or other securities; (ii) the Company shall offer for
subscription prorata to the holders of any class of its Common Stock any
additional shares of stock of any class or other rights; (iii) there shall be
any Merger Event; or (iv) there shall be any voluntary or involuntary
dissolution, liquidation or winding up of the Company; then, in connection
with each such event, the Company shall send to the Warrantholder: (A) at
least twenty (20) days' prior written notice of the date on which the books
of the Company shall close or a record shall be taken for such dividend,
distribution, subscription rights (specifying the date on which the holders
of Common Stock shall be entitled thereto) or for determining rights to vote
in respect of such Merger Event, dissolution, liquidation or winding up; and
(B) in the case of any such Merger Event, dissolution, liquidation or winding
up, at least twenty (20) days' prior written notice of the date when the same
shall take place (and specifying the date on which the holders of Common
Stock shall be entitled to exchange their Common Stock for securities or
other property deliverable upon such Merger Event, dissolution, liquidation
or winding up). In the case of a public offering, the Company shall give
Warrantholder at least twenty (20) days written notice prior to the effective
date thereof.

         Each such written notice shall set forth, in reasonable detail, (i)
the event requiring an adjustment (if any adjustment is required), (ii) the
amount of the adjustment (if any adjustment is required), (iii) the method by
which such adjustment was calculated (if any adjustment is required), (iv)
the Exercise Price after giving effect to such adjustment (if any adjustment
is required), and (v) the number of shares subject to purchase hereunder
after giving effect to such adjustment (if any adjustment is required), and
shall be given by first class mail, postage prepaid, addressed to the
Warrantholder, at the address as shown on the books of the Company.

         (g)      TIMELY NOTICE. Failure to timely provide such notice
required by subsection (f) above shall entitle Warrantholder to retain the
benefit of the applicable notice period notwithstanding anything to the
contrary contained in any insufficient notice received by Warrantholder. The
notice period shall begin on the date Warrantholder actually receives a
written notice containing all the information specified above.

9.       REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE COMPANY.

         (a)      RESERVATION OF COMMON STOCK. The Common Stock issuable upon
exercise of the Warrantholder's rights has been (or, in the case of Common
Stock issuable pursuant to Section 8(e), will be) duly and validly reserved
and, when issued in accordance with the provisions of this Warrant Agreement,
will be validly issued, fully paid and non-assessable, and will be free of
any taxes, liens, charges or encumbrances of any nature whatsoever; provided,
however, that the Common Stock issuable pursuant to this Warrant Agreement
may be subject to restrictions on transfer under state and/or Federal
securities laws. The Company has made available to the Warrantholder true,
correct


                                       6


<PAGE>

and complete copies of its Certificate, as heretofore amended. The issuance
of certificates for shares of Common Stock upon exercise of the Warrant shall
be made without charge to the Warrantholder for any issuance tax in respect
thereof, or other cost incurred by the Company in connection with such
exercise and the related issuance of shares of Common Stock. The Company
shall not be required to pay any tax which may be payable in respect of any
transfer involved and the issuance and delivery of any certificate in a name
other than that of the Warrantholder.

         (b)      DUE AUTHORITY. The execution and delivery by the Company of
this Warrant Agreement and the performance of all obligations of the Company
hereunder, including the issuance to Warrantholder of the right to acquire
the shares of Common Stock, have been duly authorized by all necessary
corporate action on the part of the Company, and this Warrant Agreement is
not inconsistent with the Company's Certificate, as amended through the
Second Re-Issue Date, or Bylaws, as amended through the Second Re-Issue Date,
does not contravene any law or governmental rule, regulation or order
applicable to it, does not and will not contravene any provision of, or
constitute a default under, any indenture, mortgage, contract or other
instrument to which it is a party or by which it is bound, and this Warrant
Agreement constitutes legal, valid and binding agreements of the Company,
enforceable in accordance with their respective terms.

         (c)      CONSENTS AND APPROVALS. No consent or approval of, giving
of notice to, registration with, or taking of any other action in respect of
any state, Federal or other governmental authority or agency is required with
respect to the execution, delivery and performance by the Company of its
obligations under this Warrant Agreement, except for the filing of notices
pursuant to Regulation D under the 1933 Act if such Regulation is being
relied upon, which filing will be effective by the time required thereby.

         (d)      ISSUED SECURITIES. All issued and outstanding shares of
Common Stock or any other securities of the Company have been duly authorized
and validly issued and are fully paid and nonassessable. All outstanding
shares of Common Stock and any other securities were issued in full
compliance with all Federal and state securities laws.

         (e)      INSURANCE. The Company has in full force and effect
insurance policies, with extended coverage, insuring the Company and its
property and business against such losses and risks, and in such amounts, as
are customary for corporations engaged in a similar business and similarly
situated and as otherwise may be required pursuant to the terms of any other
contract or agreement.

         (f)      COMMITMENTS TO REGISTER SECURITIES. As of the Second
Re-Issue Date, except as disclosed in the Company's Registration Statement,
as amended, under the 1933 Act which was declared effective by the SEC on
April 26, 2000, the Company is not, pursuant to the terms of any agreement in
existence as of the Second Re-Issue Date, under any obligation to register
under the 1933 Act any of its presently outstanding securities or any of its
securities which may hereafter be issued.


                                       7


<PAGE>

         (g)      EXEMPT TRANSACTION. Subject to the accuracy of the
Warrantholder's representations in Section 10 hereof and assuming no
circumstances at the time of the exercise of this Warrant which would render
unavailable the exemption referred to in this subparagraph (g), the issuance
of the Common Stock upon exercise of this Warrant will constitute a
transaction exempt from the registration requirements of Section 5 of the
1933 Act, in reliance upon Section 4(2) thereof.

         (h)      COMPLIANCE WITH RULE 144. At the written request of the
Warrantholder, who proposes to sell Common Stock issuable upon the exercise
of the Warrant in compliance with Rule 144 promulgated by the SEC, the
Company shall furnish to the Warrantholder, within ten days after receipt of
such request, a written statement confirming the Company's compliance with
the filing requirements of the SEC as set forth in such Rule, as such Rule
may be amended from time to time.

10.      REPRESENTATIONS AND COVENANTS OF THE WARRANTHOLDERS.

         This Warrant Agreement has been entered into by the Company in reliance
upon the following representations and covenants of the Warrantholder:

         (a)      INVESTMENT PURPOSE. The right to acquire Common Stock
represented by this Warrant Agreement and the Common Stock issuable upon
exercise of the Warrantholder's rights contained herein will be acquired for
investment and not with a view to the sale or distribution of any part
thereof, and the Warrantholder has no present intention of selling or
engaging in any public distribution of the same except pursuant to a
registration or exemption from the registration requirements of Section 5 of
the 1933 Act.

         (b)      PRIVATE ISSUE. The Warrantholder understands (i) that the
issuance of this Warrant and the Common Stock issuable upon exercise of this
Warrant is not registered under the 1933 Act or qualified under applicable
state securities laws on the ground that the issuances are or will be exempt
from the registration and qualifications requirements thereof, and (ii) that
the Company's reliance on such exemption is predicated on the representations
set forth in this Section 10. In addition, the Warrantholder consents to the
placement of a legend on any certificate or other document evidencing this
Warrant, the Common Stock issuable upon exercise of this Warrant or any
Common Stock issuable upon conversion of the Common Stock, such legend
stating that such securities have not been registered under the 1933 Act or
under any state securities or "blue sky" laws and setting forth or referring
to the restrictions on transferability and sale thereof, including the
restrictions set forth herein, until such time as such restrictions are not
longer applicable.

         (c)      DISPOSITION OF WARRANTHOLDER'S RIGHTS. In no event will the
Warrantholder make a disposition of any of its rights to acquire Common Stock
or Common Stock issuable upon exercise of such rights unless and until (i) it
shall have notified the Company of the proposed disposition, and (ii) if
requested by the Company, it shall have furnished the Company with an opinion
of counsel (which counsel may either be inside or outside counsel to the
Warrantholder) satisfactory to the Company and its counsel to the effect that
(A) appropriate action necessary for compliance with the 1933 Act has been
taken, or (B) an exemption from the registration requirements of the 1933 Act



                                       8


<PAGE>


is available. Notwithstanding the foregoing, the restrictions imposed upon
the transferability of any of its rights to acquire Common Stock or Common
Stock issuable on the exercise of such rights do not apply to transfers from
the beneficial owner of any of the aforementioned securities to its nominee
or from such nominee to its beneficial owner, and shall terminate as to any
particular share of Common Stock when (1) such security shall have been
effectively registered under the 1933 Act and sold by the holder thereof in
accordance with such registration or (2) such security shall have been sold
without registration in compliance with Rule 144 under the 1933 Act, or (3) a
letter shall have been issued to the Warrantholder at its request by the
staff of the SEC or a ruling shall have been issued to the Warrantholder at
its request by the SEC stating that no action shall be recommended by such
staff or taken by the SEC, as the case may be, if such security is
transferred without registration under the 1933 Act in accordance with the
conditions set forth in such letter or ruling and such letter or ruling
specifies that no subsequent restrictions on transfer are required. Whenever
the restrictions imposed hereunder shall terminate, as hereinabove provided,
the Warrantholder or holder of a share of Common Stock then outstanding as to
which such restrictions have terminated shall be entitled to receive from the
Company, without expense to such holder, one or more new certificates for the
Warrant or for such shares of Common Stock not bearing any restrictive legend.

         (d)      FINANCIAL RISK. The Warrantholder has such knowledge and
experience in financial and business matters as to be capable of evaluating
the merits and risks of its investment, and has the ability to bear the
economic risks of its investment.

         (e)      RISK OF NO REGISTRATION. The Warrantholder understands that
if the Company does not register with the SEC pursuant to Section 12 of the
1933 Act, or file reports pursuant to Section 15(d) of the Securities
Exchange Act of 1934, as amended, or if a registration statement covering the
securities under the 1933 Act is not in effect when it desires to sell (i)
the rights to purchase Common Stock pursuant to this Warrant Agreement, or
(ii) the Common Stock issuable upon exercise of the right to purchase, it may
be required to hold such securities for an indefinite period. The
Warrantholder also understands that any sale of its rights as the
Warrantholder to purchase Common Stock or Common Stock which might be made by
it in reliance upon Rule 144 under the 1933 Act may be made only in
accordance with the terms and conditions of that Rule.

11.      TRANSFERS.

         Subject to the terms and conditions contained in Section 10 hereof,
this Warrant Agreement and all rights hereunder are transferable in whole or
in part by the Warrantholder and any successor transferee, provided, however,
in no event shall the number of transfers of the rights and interests in all
of the Warrants exceed a total of three (3) transfers. The transfer shall be
recorded on the books of the Company upon receipt by the Company of a notice
of transfer in the form attached hereto as Exhibit III (the "Transfer
Notice"), at its principal offices and the payment to the Company of all
transfer taxes and other governmental charges imposed on such transfer.



                                       9


<PAGE>

12.      MISCELLANEOUS.

         (a)      EFFECTIVE DATE. The provisions of this Warrant Agreement
(except for references to instruments dated after the Effective Date) shall
be construed and shall be given effect in all respects as if it had been
executed and delivered by the Company as of the Effective Date. This Warrant
Agreement shall be binding upon any successors or assigns of the Company.

         (b)      ATTORNEY'S FEES. In any litigation, arbitration or court
proceeding between the Company and the Warrantholder relating hereto, the
prevailing party shall be entitled to attorneys' fees and expenses and all
costs of proceedings incurred in enforcing this Warrant Agreement.

         (c)      GOVERNING LAW. This Warrant Agreement shall be governed by
and construed for all purposes under and in accordance with the laws of the
State of Illinois.

         (d)      COUNTERPARTS. This Warrant Agreement may be executed in
counterparts, each of which shall be deemed an original, but both of which
together shall constitute one and the same instrument.

         (e)      NOTICES. Any notice required or permitted hereunder shall
be given in writing and shall be deemed effectively given upon personal
delivery, facsimile transmission (provided that the original is sent by
personal delivery or mail as hereinafter set forth) or seven (7) days after
deposit in the United States mail, by registered or certified mail, addressed
(i) to the Warrantholder at 49 Tanglewood Road, Wellesley, Massachusetts
02481, attention: Gregory Stento, and (ii) to the Company at 1 Hampshire
Street, Cambridge, Massachusetts 02139-1572, attention: President (and/or if
by facsimile, (617) 494-8414) or at such other address as any such party may
subsequently designate by written notice to the other party.

         (f)      REMEDIES. In the event of any default hereunder, the
non-defaulting party may proceed to protect and enforce its rights either by
suit in equity and/or by action at law, including but not limited to an
action for damages as a result of any such default, and/or an action for
specific performance for any default where a party will not have an adequate
remedy at law and where damages will not be readily ascertainable. The
Company expressly agrees that it shall not oppose, on the grounds that on
adequate remedy at law exists, an application by the Warrantholder or any
other person entitled to the benefit of this Agreement requiring specific
performance of any or all provisions hereof or enjoining the Company from
continuing to commit any such breach of this Agreement.

         (g)      NO IMPAIRMENT OF RIGHTS. The Company will not, by amendment
of its Certificate or through any other means, avoid or seek to avoid the
observance or performance of any of the terms of this Warrant, but will at
all times in good faith assist in the carrying out of all such terms.


                                       10

<PAGE>


         (h)      SURVIVAL. The representations, warranties, covenants and
conditions of the respective parties contained herein or made pursuant to this
Warrant Agreement shall survive the execution and delivery of this Warrant
Agreement.

         (i)      SEVERABILITY. In the event any one or more of the
provisions of this Warrant Agreement shall for any reason be held invalid,
illegal or unenforceable, the remaining provisions of this Warrant Agreement
shall be unimpaired, and the invalid, illegal or unenforceable provision
shall be replaced by a mutually acceptable valid, legal and enforceable
provision, which comes closest to the intention of the parties underlying the
invalid, illegal or unenforceable provision.

         (j)      AMENDMENTS. Any provision of this Warrant Agreement may be
amended by a written instrument signed by the Company and by the
Warrantholder.

         (k)      ADDITIONAL DOCUMENTS. The Company shall supply such
documents as may be reasonably necessary to confirm the Company's compliance
with its obligations hereunder (subject to reasonable confidentiality
restrictions) as the Warrantholder may from time to time reasonably request.

         IN WITNESS WHEREOF, the parties hereto have caused this Warrant
Agreement to be executed by its officers thereunto duly authorized as of the
Effective Date.

                                    Company: PRAECIS PHARMACEUTICALS
                                    INCORPORATED

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

                                    Warrantholder: Gregory Stento

                                         /s/ Gregory Stento
                                     --------------------------------


                                       11


<PAGE>



                                    EXHIBIT I

                               NOTICE OF EXERCISE

To:_______________________

(1)      The undersigned Warrantholder hereby elects to purchase
         _______________________________________ shares of the Common Stock of
         PRAECIS PHARMACEUTICALS INCORPORATED (the "Company"), pursuant to the
         terms of the Warrant Agreement effective as of July 31, 2000 between
         the Company and the undersigned, as transferee of the rights of
         Comdisco, Inc. ("Comdisco"), with respect to the purchase of up to
         9,860 shares of the Company's Common Stock, originally granted to
         Comdisco pursuant to the terms of the Warrant Agreement originally
         dated as of March 29, 1995, reissued as of August 12, 1998, and
         reissued again as of July 31, 2000 (the "Warrant Agreement"), between
         the Company and Gregory Stento and tenders herewith payment of the
         purchase price for such shares in full, together with all applicable
         transfer taxes, if any.

(2)      In exercising its rights to purchase the Common Stock of the Company,
         the undersigned hereby confirms and acknowledges the investment
         representations and warranties made in Section 10 of the Warrant
         Agreement.

(3)      Please issue a certificate or certificates representing said shares of
         Common Stock in the name of the undersigned or in such other name as is
         specified below.


____________________
(Name)

____________________
(Address)


Warrantholder: GREGORY STENTO


_____________________


Date:________________



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<PAGE>



                                                     EXHIBIT II

                                             ACKNOWLEDGMENT OF EXERCISE

         The undersigned ________________________________     , hereby
acknowledges receipt of the "Notice of Exercise" from Gregory Stento, to
purchase _______     shares of the Common Stock of PRAECIS PHARMACEUTICALS
INCORPORATED pursuant to the terms of the Warrant Agreement, and further
acknowledges that _________     shares remain subject to purchase under the
terms of the Warrant Agreement.

                                     Company: PRAECIS PHARMACEUTICALS
                                              INCORPORATED

                                     By:_____________________________

                                     Title:__________________________

                                     Date:___________________________



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<PAGE>



                                   EXHIBIT III

                                 TRANSFER NOTICE

(To transfer or assign the foregoing Warrant Agreement execute this form and
supply required information. Do not use this form to purchase shares.)

         FOR VALUE RECEIVED, the foregoing Warrant Agreement and all rights
evidenced thereby are hereby transferred and assigned to


___________________________________________________________________________
                                 (Please Print)

whose address is___________________________________________________________
___________________________________________________________________________


                               Dated_______________________________________

                               Holder's Signature__________________________

                               Holder's Address____________________________


___________________________________________________________________________
Signature Guaranteed:______________________________________________________



      NOTE:               The signature to this Transfer
                          Notice must correspond with the name as it
                          appears on the face of the Warrant
                          Agreement, without alteration or enlargement
                          or any change whatever. Officers of
                          corporations and those acting in a fiduciary
                          or other representative capacity should file
                          proper evidence of authority to assign the
                          foregoing Warrant Agreement.





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