<PAGE>

                                                          EXHIBIT 10.21

                              PHARSIGHT CORPORATION

                            STOCK OPTION GRANT NOTICE

            (2000 CEO NON-QUALIFIED STOCK OPTION PLAN AND AGREEMENT)

PHARSIGHT CORPORATION, a California corporation (the "Company"), pursuant to its
2000 CEO Non-Qualified Stock Option Plan and Agreement (the "CEO Plan and
Agreement") and this Stock Option Grant Notice (the "Grant Notice"), hereby
grants to Optionee an option (the "Option") to purchase the number of shares of
the Company's common stock set forth below (the "Shares"). This Option is
subject to all of the terms and conditions as set forth herein and in the CEO
Plan and Agreement and the Notice of Exercise, all of which are attached hereto
and incorporated herein in their entirety.

          OPTIONEE:                           Arthur H. Reidel

          OPTION GRANT NUMBER:                OP-267

          TYPE OF OPTION:                     Non-Qualified Stock Option

          DATE OF GRANT:                      May 16, 2000

          SHARES SUBJECT TO OPTION:           442,750

          EXERCISE PRICE PER SHARE:           $6.83

          EXPIRATION DATE:                    May 15, 2008

          VESTING SCHEDULE:                   The Shares shall vest as follows:
                                              one thirty-fourth (1/34th) of the
                                              Shares shall vest on June 1, 2001;
                                              one thirty-fourth (1/34th) of the
                                              Shares shall vest monthly on the
                                              first day of each month for each
                                              of the next thirty-three (33)
                                              months.


          EXERCISE SCHEDULE:                  Same as "Vesting Schedule."  Early
                                              Exercise is not permitted.

PAYMENT: Payment of the Option exercise price may be made in cash or check or by
any other method provided in the CEO Plan and Agreement.

ADDITIONAL TERMS/ACKNOWLEDGEMENTS: The undersigned Optionee acknowledges receipt
of, and understands and agrees to, this Grant Notice and the attached CEO Plan
and Agreement. Optionee further acknowledges that as of the Date of Grant, this
Grant Notice, and the CEO Plan and Agreement, set forth the entire understanding
between Optionee and the Company regarding the acquisition of stock in the
Company and supersedes all prior oral and written agreements on that subject. At
the time the Option is exercised, in whole or in part, or at any time thereafter
as requested by the Company, Optionee hereby authorizes withholding from payroll
and any other amounts payable to him, and otherwise agrees to make adequate
provision for (including by means of a "cashless exercise" pursuant to a program
developed under Regulation T (or similar rule or regulation) as promulgated by
the Federal Reserve Board), any sums required to satisfy the federal, state,
local and foreign tax withholding obligations of the Company or an Affiliate (as
defined in the CEO Plan and Agreement), if any, which arise in connection with
the Option.

PHARSIGHT CORPORATION                         OPTIONEE:  ARTHUR H. REIDEL

By:           /s/ Robin A. Kehoe                  /s/ Arthur H. Reidel
    -----------------------------------       -------------------------------
Name:         Robin A. Kehoe                            Signature
     ----------------------------------
Title:        Chief Financial Officer
     ----------------------------------
Date:         June 15, 2000
     ----------------------------------

Attachments: 2000 CEO Non-Qualified Stock Option Plan and Agreement, Notice of
Exercise


<PAGE>

                              PHARSIGHT CORPORATION
             2000 CEO NON-QUALIFIED STOCK OPTION PLAN AND AGREEMENT

     Pursuant to this 2000 CEO Non-Qualified Stock Option Plan and Agreement
(the "CEO Plan and Agreement") and the Stock Option Grant Notice ("Grant
Notice") and the Notice of Exercise, PHARSIGHT CORPORATION, a California
corporation (the "Company"), hereby grants a non-qualified stock option (the
"Option") to the Company's Chief Executive Officer to purchase the number of
shares of the common stock of the Company (the "Common Stock") indicated in the
Grant Notice (the "Option Shares") at the exercise price indicated in the Grant
Notice. The Option shall be effective on the date of grant specified in the
Grant Notice.

     The Option is not intended to qualify as an "incentive stock option" within
the meaning of Section 422 of the Internal Revenue Code of 1986, as amended (the
"Code") The Option is intended to comply with the exemption from qualification
provided by Section 25102(f) of the California Corporations Code. The Option
Shares covered by the grant of this Option are intended to be exempt from
registration pursuant to Section 4(2) of the Securities Act of 1933, as amended
(the "Securities Act"), but specifically not in reliance on Rule 701 promulgated
thereunder.

     1. ELIGIBILITY. Subject to Section 10 related to transferability, the sole
person eligible to receive an Option under the CEO Plan and Agreement is Arthur
H. Reidel, the Company's Chief Executive Officer (the "Optionee").

     2. SHARES SUBJECT TO THE CEO PLAN AND AGREEMENT. Subject to the provisions
of Section 11 relating to adjustments upon changes in stock, the stock that may
be issued under the CEO Plan and Agreement shall not exceed in the aggregate
Four Hundred Forty-Two Thousand Seven Hundred Fifty (442,750) shares of Common
Stock. If the Option shall for any reason expire or otherwise terminate, in
whole or in part, without having been exercised in full, the stock not acquired
under the Option shall not revert to and shall not again become available for
issuance under the CEO Plan and Agreement. At all times, the Company shall
reserve and keep available a sufficient number of shares of Common Stock as will
be required to satisfy the requirements of the Option granted under this CEO
Plan and Agreement.

     3. NUMBER OF SHARES AND EXERCISE PRICE.

          (a) The total number of shares of Common Stock subject to the Option
is equal to the Option Shares.

          (b) The exercise price per share of Common Stock shall be the price
provided on the Grant Notice, which price shall be one hundred five percent
(105%) of the Fair Market Value (as defined below) on the date of grant.

          (c) For purposes of the Option, "Fair Market Value" means, as of any
date, the value of the Common Stock determined as follows:

               (i) If the Common Stock is listed on any established stock
exchange or traded on the Nasdaq National Market or the Nasdaq SmallCap Market,
the Fair Market Value


<PAGE>

of a share of Common Stock shall be the closing sales price for such stock (or
the closing bid, if no sales were reported) as quoted on such exchange or market
(or the exchange or market with the greatest volume of trading in the Common
Stock) on the last market trading day prior to the day of determination, as
reported in THE WALL STREET JOURNAL or such other source as the Board of
Directors (the "Board") deems reliable.

               (ii) In the absence of such markets for the Common Stock, the
Fair Market Value shall be determined in good faith by the Board.

     4. VESTING AND EXERCISABILITY. Subject to the limitations contained herein,
the Option shall vest as provided in Grant Notice, provided that vesting will
cease upon the termination of the Optionee's continuous service with the Company
or of any Affiliate of the Company as an employee (hereinafter, "Continuous
Service"). For purposes of this CEO Plan and Agreement, Affiliate means any
parent corporation or subsidiary corporation of the Company as such terms are
defined in Sections 424(e) and (f) respectively of the Code. On all exercises,
fractions of shares shall be rounded to the next lowest number. The Option will
be exercisable only to the extent the Option Shares have vested.

     5. METHOD OF PAYMENT. Payment of the exercise price is due in full upon
exercise of all or any part of the Option. The Optionee may elect to make
payment of the exercise price in cash or by check or in any other manner that is
specified in the Grant Notice, which may include one or more of the following:

          (a) In the Company's sole discretion at the time the Option is
exercised and provided that at the time of exercise the Common Stock is publicly
traded and quoted regularly in THE WALL STREET JOURNAL, pursuant to a program
developed under Regulation T (or similar rule or regulation) as promulgated by
the Federal Reserve Board that, prior to the issuance of Common Stock, results
in either the receipt of cash (or check) by the Company or the receipt of
irrevocable instructions to pay the aggregate exercise price to the Company from
the sales proceeds.

          (b) Provided that at the time of the exercise of the Option the Common
Stock is publicly traded and quoted regularly in THE WALL STREET JOURNAL, by
delivery of already-owned shares of Common Stock, held for the period required
to avoid a charge to the Company's reported earnings (generally six months) or
that were not acquired, directly or indirectly from the Company, that are owned
free and clear of any liens, claims, encumbrances or security interests, and
valued at its Fair Market Value (as defined below) on the date of exercise.
"Delivery" for these purposes, in the sole discretion of the Company at the time
of the exercise of the Option, shall include delivery to the Company of the
Optionee's attestation of ownership of such shares of Common Stock in a form
approved by the Company. Notwithstanding the foregoing, the Option may not be
exercised by tender to the Company of Common Stock to the extent such tender
would constitute a violation of the provisions of any law, regulation or
agreement restricting the redemption of the Company's stock.

          (c) Pursuant to the following deferred payment alternative:


                                      -2-
<PAGE>


               (i) Not less than one hundred percent (100%) of the aggregate
exercise price, plus accrued interest, shall be due four (4) years from date of
exercise or, at the Company's election, upon earlier termination of the
Optionee's Continuous Service with the Company or an Affiliate of the Company.

               (ii) Interest shall be paid at least annually and shall be
charged at the minimum rate of interest necessary to avoid the treatment as
interest, under any applicable provisions of the Code, of any portion of any
amounts other than amounts stated to be interest under the deferred payment
arrangement.

               (iii) At any time that the Company is incorporated in Delaware,
payment of the Common Stock's "par value," as defined in the Delaware General
Corporation Law, shall be made in cash and not by deferred payment.

               (iv) In order to elect the deferred payment alternative, the
Optionee must, as a part of his written notice of exercise, give notice of the
election of this payment alternative and, in order to secure the payment of the
deferred exercise price to the Company hereunder, if the Company so requests,
the Optionee must tender to the Company a full recourse promissory note and a
security agreement covering the purchased shares, both in form and substance
satisfactory to the Company, or such other or additional documentation as the
Company may request.

     6. WHOLE SHARES. The Option may only be exercised for whole shares.

     7. SECURITIES LAW COMPLIANCE. Notwithstanding anything to the contrary
contained herein, the Option may not be exercised unless the shares issuable
upon exercise of the Option are then registered under the Securities Act or, if
such shares are not then so registered, the Company has determined that such
exercise and issuance would be exempt from the registration requirements of the
Securities Act. The exercise of the Option must also comply with other
applicable laws and regulations governing the Option, and the Option may not be
exercised if the Company determines that the exercise would not be in material
compliance with such laws and regulations.

     8. TERM. The term of the Option commences on the Date of Grant and expires
upon the EARLIEST of the following:

          (a) three (3) months after the termination of Continuous Service for
any reason other than Disability (as defined below) or death, provided that if
during any part of such three (3) month period the Option is not exercisable
solely because of the condition set forth in Section 7, the Option shall not
expire until the earlier of the expiration date specified in the Grant Notice
(the "Expiration Date") or until it shall have been exercisable for an aggregate
period of three (3) months after the termination of the Optionee's Continuous
Service;

          (b) twelve (12) months after the termination of Optionee's Continuous
Service due to Optionee's Disability (as defined below).


                                      -3-
<PAGE>

          (c) eighteen (18) months after the Optionee's death if Optionee dies
either during Optionee's Continuous Service or within three (3) months after
Optionee's Continuous Service terminates;

          (d) the Expiration Date indicated in the Grant Notice; or

          (e) the day before the eight (8th) anniversary of the Date of Grant.

     For purposes of this CEO Plan and Agreement, "Disability" means the
inability of the Optionee, in the opinion of a qualified physician acceptable to
the Company, to perform the major duties of Optionee's position with the Company
or an Affiliate of the Company because of sickness or injury of the person.

     9. EXERCISE.

          (a) The Optionee may exercise the vested portion of the Option during
its term by delivering a Notice of Exercise (in a form designated by the
Company) together with the exercise price to the Secretary of the Company, or to
such other person as the Company may designate, during regular business hours,
together with such additional documents as the Company may then require.

          (b) As a condition to any exercise of the Option, the Company may
require the Optionee to enter an arrangement providing for the payment by the
Optionee to the Company of any tax withholding obligation of the Company arising
by reason of (i) the exercise of the Option, (ii) the lapse of any substantial
risk of forfeiture to which the shares are subject at the time of exercise, or
(iii) the disposition of shares acquired upon such exercise.

          (c) The Company (or a representative of the underwriters) may, in
connection with the first underwritten registration of the offering of any
securities of the Company under the Securities Act, require that the Optionee
not sell, dispose of, transfer, make any short sale of, grant any option for the
purchase of, or enter into any hedging or similar transaction with the same
economic effect as a sale, any shares of Common Stock or other securities of the
Company held by the Optionee, for a period of time specified by the
underwriter(s) (not to exceed one hundred eighty (180) days) following the
effective date of the registration statement of the Company filed under the
Securities Act. The Optionee further agrees to execute and deliver such other
agreements as may be reasonably requested by the Company and/or the
underwriter(s) which are consistent with the foregoing or which are necessary to
give further effect thereto. In order to enforce the foregoing covenant, the
Company may impose stop-transfer instructions with respect to the Optionee's
Common Stock until the end of such period.

     10. TRANSFERABILITY. The Option is not transferable, except by will or by
the laws of descent and distribution, and is exercisable during the Optionee's
life only by the Optionee. Notwithstanding the foregoing, by delivering written
notice to the Company, in a form satisfactory to the Company, the Optionee may
designate a third party who in the event of the Optionee's death shall
thereafter be entitled to exercise the Option subject to all of the terms and
conditions herein.


                                      -4-
<PAGE>


     11. ADJUSTMENTS UPON CHANGES IN STOCK

          (a) CAPITALIZATION ADJUSTMENTS. If any change is made in the Common
Stock subject to the CEO Plan and Agreement, without the receipt of
consideration by the Company (through merger, consolidation, reorganization,
recapitalization, reincorporation, stock dividend, dividend in property other
than cash, stock split, liquidating dividend, combination of shares, exchange of
shares, change in corporate structure or other transaction not involving the
receipt of consideration by the Company), the CEO Plan and Agreement will be
appropriately adjusted in the class(es) and maximum number of securities subject
to the CEO Plan and Agreement pursuant to Section 2 and the maximum number of
securities subject to the Option will be appropriately adjusted in the class(es)
and number of securities and price per share of Common Stock. The Board shall
make such adjustments, and its determination shall be final, binding and
conclusive. (The conversion of any convertible securities of the Company shall
not be treated as a transaction "without receipt of consideration" by the
Company.)

          (b) DISSOLUTION OR LIQUIDATION. In the event of a dissolution or
liquidation of the Company, then the Option shall terminate immediately prior to
such event.

          (c) ASSET SALE, MERGER, CONSOLIDATION OR REVERSE MERGER. In the event
of (i) a sale, lease or other disposition of all or substantially all of the
assets of the Company, (ii) a merger or consolidation in which the Company is
not the surviving corporation or (iii) a reverse merger in which the Company is
the surviving corporation but the shares of Common Stock outstanding immediately
preceding the merger are converted by virtue of the merger into other property,
whether in the form of securities, cash or otherwise (individually, a "Corporate
Transaction"), then any surviving corporation or acquiring corporation may
assume the Option or may substitute a similar stock award (including an award to
acquire the same consideration paid to the shareholders in the Corporate
Transaction) for the Option. In the event any surviving corporation or acquiring
corporation does not assume the Option or substitute a similar stock award for
the Option, provided the Optionee is then providing Continuous Service, then the
vesting of the Option (and the time during which Option may be exercised) shall
be accelerated in full, and the Option shall terminate if not exercised at or
prior to the Corporate Transaction.

     12. REPRESENTATIONS.

     (a) By executing the Grant Notice, Optionee hereby warrants and
represents that Optionee is acquiring the Option for Optionee's own account
and that Optionee has no intention of distributing, transferring or selling
all or any part of the Option except in accordance with the terms of the CEO
Plan and Agreement and Section 25102(f) of the California Corporations Code.
Optionee also hereby warrants and represents that Optionee has either (i)
preexisting personal or business relationships with the Company or any of its
officers, directors or controlling persons, or (ii) the capacity to protect
Optionee's own interests in connection with the grant of the Option by virtue
of Optionee's business or financial expertise or the business or financial
expertise of any of Optionee's professional advisors who are unaffiliated
with and who are not compensated by the Company or any of its Affiliates,
directly or indirectly.

     (b) The Company may require Optionee, as a condition of exercising or
acquiring Common Stock under the Option, (i) to give written assurances
satisfactory to the


                                      -5-
<PAGE>

Company as to the Optionee's knowledge and experience in financial and business
matters and/or to employ a purchaser representative reasonably satisfactory to
the Company who is knowledgeable and experienced in financial and business
matters and that he or she is capable of evaluating, alone or together with the
purchaser representative, the merits and risks of exercising the Option; and
(ii) to give written assurances satisfactory to the Company stating that
Optionee is acquiring Common Stock subject to the Option for Optionee's own
account and not with any present intention of selling or otherwise distributing
the Common Stock. The foregoing requirements, and any assurances given pursuant
to such requirements, shall be inoperative if (1) the issuance of the shares of
Common Stock upon exercise of the Option has been registered under a then
current effective registration statement under the Securities Act or (2) as to
any particular requirement, a determination is made by counsel for the Company
that such requirement need not be met in the circumstances under the then
applicable securities laws. The Company may, upon advice of counsel to the
Company, place legends on stock certificates issued under the CEO Plan and
Agreement as such counsel deems necessary or appropriate in order to comply with
applicable securities laws, including, but not limited to, legends restricting
the transfer of the Common Stock.

     13. RIGHT OF REPURCHASE. To the extent provided in the Company's bylaws as
amended from time to time, the Company shall have the right to repurchase all or
any part of the shares of Common Stock received pursuant to the exercise of the
Option. The Company's right of repurchase shall expire on the date of the first
registration of the Common Stock under Section 12 of the Securities and Exchange
Act of 1934, as amended (the "Exchange Act").

     14. RIGHT OF FIRST REFUSAL. Shares of Common Stock that Optionee acquires
upon exercise of the Option are subject to any right of first refusal that may
be described in the Company's bylaws in effect at such time the Company elects
to exercise its right. The Company's right of first refusal shall expire on the
date of the first registration of the Common Stock under Section 12 of the
Exchange Act.

     15. OPTION NOT A SERVICE CONTRACT. Neither the Option nor the CEO Plan and
Agreement nor the Grant Notice constitutes an employment or service contract,
and nothing in the Option shall be deemed to create in any way whatsoever any
obligation on the Optionee's part to continue in the employ of the Company or an
Affiliate, or of the Company or an Affiliate to continue the Optionee's
employment. In addition, nothing in the Option shall obligate the Company or an
Affiliate, their respective shareholders, Boards of Directors, officers or
employees to continue any relationship that the Optionee might have as a
director or consultant for the Company or an Affiliate.

     16. WITHHOLDING OBLIGATIONS.

    (a) At the time Optionee exercises the Option, in whole or in part, or at
any time thereafter as requested by the Company, the Optionee hereby
authorizes withholding from payroll and any other amounts payable to Optionee
and otherwise agrees to make adequate provisions for (including by means of a
"cashless exercise" pursuant to a program developed under Regulation T (or
similar rule or regulation) as promulgated by the Federal Reserve Board to
the extent permitted by the Company) any sums required to satisfy the
federal, state, local and

                                      -6-
<PAGE>

foreign tax withholding obligations of the Company or any Affiliate, if any,
which arise in connection with the Option.

          (b) Upon the Optionee's request and subject to approval by the
Company, in its sole discretion, and compliance with any applicable conditions
or restrictions of law, the Company may withhold from fully vested shares of
Common Stock otherwise issuable to the Optionee upon the exercise of the Option
a number of whole shares of Common Stock having a Fair Market Value, determined
as of the date of exercise, not in excess of the minimum amount of tax required
to be withheld by law. If the date of determination of any tax withholding
obligation would be deferred to a date later than the date of exercise of the
Option, withholding of shares of Common Stock pursuant to the preceding sentence
shall not be permitted unless the Optionee makes a proper and timely election
under Section 83(b) of the Code, covering the aggregate number of shares of
Common Stock acquired upon such exercise with respect to which such
determination is otherwise deferred, to accelerate the determination of such tax
withholding obligation to the date of exercise of the Option. Notwithstanding
the filing of such election, shares of Common Stock shall be withheld solely
from fully vested shares of Common Stock determined as of the date of exercise
of the Option that are otherwise issuable to the Optionee upon such exercise.
Any adverse consequences to the Optionee arising in connection with such share
withholding procedure shall be the Optionee's sole responsibility.

          (c) The Option is not exercisable unless the tax withholding
obligations of the Company and/or any Affiliate are satisfied. Accordingly, the
Optionee may not be able to exercise the Option when desired even though the
Option is vested, and the Company shall have no obligation to issue a
certificate for such shares or release such shares from any escrow provided for
herein.

     17. NOTICES. Any notices provided for in the CEO Plan and Agreement shall
be given in writing and shall be deemed effectively given upon receipt or, in
the case of notices delivered by the Company to the Optionee, five (5) days
after deposit in the United States mail, postage prepaid, addressed to the
Optionee at the last address the Optionee provided to the Company.

     18. GOVERNING PLAN AND AGREEMENT DOCUMENT. The Option is subject to all the
provisions of the CEO Plan and Agreement, Grant Notice and Notice of Exercise,
and is further subject to all interpretations, amendments, rules and regulations
which may from time to time be promulgated and adopted pursuant to the CEO Plan
and Agreement. With respect to (i) any and all matters relating to the Option
granted under this CEO Plan and Agreement, Grant Notice and Notice of Exercise,
(ii) any Option shares and (iii) any shares of Common Stock to be received upon
exercise of such Option, the terms and conditions of the CEO Plan and Agreement,
Grant Notice and Notice of Exercise shall govern at all times.


                                      -7-
<PAGE>

     19. ADMINISTRATION BY THE BOARD.

          (a) The CEO Plan and Agreement shall be administered by the Board,
unless and until the Board delegates administration to a committee as provided
in subsection (b). The Board shall have the authority to construe and interpret
the CEO Plan and Agreement and to establish, amend or waive rules and
regulations for its administration. All determinations, interpretations and
constructions made by the Board in good faith shall not be subject to review by
any person and shall be final, binding and conclusive on all persons.

          (b) The Board may delegate administration of the CEO Plan and
Agreement to a committee of one (1) or more members of the Board, and the term
"Committee" shall apply to any person or persons to whom such authority has been
delegated. If administration is delegated to a Committee, the Committee shall
have, in connection with the administration of the CEO Plan and Agreement, the
powers possessed by the Board, subject to such resolutions, not inconsistent
with the provisions of the CEO Plan and Agreement, as may be adopted from time
to time by the Board. The Board may abolish the Committee at any time and revest
in the Board the administration of the CEO Plan and Agreement. At such time as
the common stock of the Company is publicly traded, at the discretion of the
Board, a Committee may consist solely of two or more Outside Directors (as
defined below), in accordance with Section 162(m) of the Code, and/or solely of
two or more Non-Employee Directors (as defined below), in accordance with Rule
16b-3.

     For purposes of this CEO Plan and Agreement, "Outside Director" means a
director who either (i) is not a current employee of the Company or an
"affiliated corporation" (within the meaning of Treasury Regulations promulgated
under Section 162(m) of the Code), is not a former employee of the Company or an
"affiliated corporation" receiving compensation for prior services (other than
benefits under a tax qualified pension plan), was not an officer of the Company
or an "affiliated corporation" at any time and is not currently receiving direct
or indirect remuneration from the Company or an "affiliated corporation" for
services in any capacity other than as a director or (ii) is otherwise
considered an "outside director" for purposes of Section 162(m) of the Code.

     For purposes of this CEO Plan and Agreement, "Non-Employee Director" means
a director who either (i) is not a current employee or officer of the Company or
its parent or a subsidiary, does not receive compensation (directly or
indirectly) from the Company or its parent or a subsidiary for services rendered
as a consultant or in any capacity other than as a director (except for an
amount as to which disclosure would not be required under Item 404(a) of
Regulation S-K promulgated pursuant to the Securities Act ("Regulation S-K")),
does not possess an interest in any other transaction as to which disclosure
would be required under Item 404(a) of Regulation S-K and is not engaged in a
business relationship as to which disclosure would be required under Item 404(b)
of Regulation S-K or (ii) is otherwise considered a "non-employee director" for
purposes of Rule 16b-3.

     20. WAIVER. The waiver by either party herein of a breach of any provision
of this CEO Plan and Agreement shall not operate or be construed as a waiver of
any other or subsequent breach.


                                      -8-
<PAGE>


     21. BINDING ON SUCCESSORS. This CEO Plan and Agreement shall inure to the
benefit of and be binding upon the parties hereto and, to the extent not
prohibited herein, their respective heirs and successors.

     22. COUNTERPARTS. This CEO Plan and Agreement may be executed in two (2) or
more counterparts, each of which shall be deemed an original but all of which
together shall constitute one and the same instrument.

     23. GOVERNING LAW. The Grant Notice, CEO Plan and Agreement, and the Notice
of Exercise shall be construed in accordance with and governed by the laws of
the State of California without giving effect to the doctrine of conflict of
laws.

     24. EFFECTIVE DATE AND DURATION OF THE CEO PLAN AND AGREEMENT. The CEO Plan
and Agreement shall be effective as of May 16, 2000. The Board may suspend or
terminate the CEO Plan and Agreement at any time. Unless sooner terminated, the
CEO Plan and Agreement shall terminate at midnight on May 15, 2008 .

     25. NO RIGHTS AS SHAREHOLDER. The Optionee shall have no rights as a
shareholder with respect to any stock subject to the Option prior to the date of
exercise and until the Optionee has satisfied all requirements for exercise of
the Option pursuant to its terms.

Dated as of the 16 day of May, 2000.

                                      Very truly yours,

                                      PHARSIGHT CORPORATION

                                      By:      /s/ Robin A. Kehoe
                                          ----------------------------------
                                               Duly authorized on behalf of
                                                  the Board of Directors

                                      Name:    Robin A. Kehoe
                                          ----------------------------------
                                      Title:      Chief Financial Officer
                                          ----------------------------------
                                      OPTIONEE

                                                /s/ Arthur H. Reidel
                                      --------------------------------------
                                      Arthur H. Reidel

                                      Address:

                                      P. O. Box 61030
                                      --------------------------------------
                                      Palo Alto, CA  94306
                                      --------------------------------------

                                      --------------------------------------

                                      --------------------------------------

<PAGE>

                               NOTICE OF EXERCISE

PHARSIGHT CORPORATION
800 West El Camino Real
Suite 200
Mountain View, California  94040

                                                              Date of Exercise:

Ladies and Gentlemen:

         This constitutes notice under my Option that I elect to purchase the
number of shares for the price set forth below.

         Stock option dated:
                                                              -----------------

         Option Grant Number:                                 OP-267

         Number of shares as
         to which option is
         exercised:
                                                              -----------------
         Certificate to be
         issued in name of:                                   Arthur H. Reidel

         Total exercise price:                                $
                                                              -----------------
         Cash payment (or check)
         delivered herewith:                                  $
                                                              -----------------
         Promissory Note delivered
         herewith:                                            $
                                                              -----------------
         Value of Pharsight Corporation
         shares of Common Stock delivered
         herewith(1):                                         $
                                                              -----------------

         By this exercise, I agree (i) to provide such additional documents as
you may require pursuant to the terms of the 2000 CEO Non-Qualified Stock Option
Plan and Agreement and (ii) to provide for the payment by me to you (in the
manner designated by you) of your withholding obligation, if any, relating to
the exercise of this option.

--------------
(1) Shares must be valued in accordance with the terms of the option being
exercised, must have been owned for the minimum period required in the
option, and must be owned free and clear of any liens, claims, encumbrances
or security interests. Certificates must be endorsed or accompanied by an
executed assignment separate from certificate.

<PAGE>

         I hereby make the following statements with respect to the shares of
Common Stock (the "Shares"), which are being acquired by me for my own account
upon this exercise of the option as set forth above:

         I warrant and represent that I am acquiring the Shares for my own
account and that I have no intention of distributing, transferring or selling
all or any part of the Shares except in accordance with the terms of the option
agreement and Section 25102(f) of the California Corporations Code. I also
hereby warrant and represent that I have either (i) preexisting personal or
business relationships with the Company or any of its officers, directors or
controlling persons, or (ii) the capacity to protect my own interests in
connection with the sale of the Shares by virtue of my own business or financial
expertise or the business or financial expertise of my professional advisors who
are unaffiliated with and who are not compensated by the Company or any of its
affiliates, directly or indirectly.

         I acknowledge that the Shares have not been registered under the
Securities Act of 1933, as amended (the "Securities Act"), and are deemed to
constitute "restricted securities" under Rule 144 promulgated under the
Securities Act. I am aware that among the conditions imposed on the transfer of
the Shares is the availability of current information to the public about the
Company and that the Company has not made such information available and has no
present plans to do so. I warrant and represent to the Company that I have no
present intention of distributing or selling said Shares, except as permitted
under the Securities Act and any applicable state securities laws.

         I acknowledge and agree that the Shares being acquired by me must be
held indefinitely unless they are subsequently registered under the Securities
Act or an exemption from such registration is available. I acknowledge and agree
that the Company has no obligation to register the Shares or to comply with any
exemption from such registration. I acknowledge and agree that under Rule 144,
as an affiliate of the Company, I will not be able to resell the Shares without
observing all restrictions imposed by Rule 144.

         I acknowledge and agree that all certificates representing any of the
Shares subject to the provisions of the option shall have endorsed thereon
appropriate legends reflecting the foregoing limitations, as well as any legends
reflecting restrictions pursuant to the Company's Articles of Incorporation,
Bylaws and/or applicable securities laws.

         I further agree that, if required by the Company (or a representative
of the underwriters) in connection with the first underwritten registration of
the offering of any securities of the Company under the Securities Act, I will
not to sell, dispose of, transfer, make any short sale of, grant any option for
the purchase of, or enter into any hedging or similar transaction with the same
economic effect as a sale, any Shares or other securities of the Company held by
me, for a period of time specified by the underwriter(s) (not to exceed one
hundred eighty (180) days) following the effective date of a registration
statement of the Company filed under the Securities Act. I further agree to
execute and deliver such other agreements as may be reasonably requested by the
Company and/or the underwriter(s) which are consistent with the foregoing or
which are necessary to give further effect thereto. In order to enforce the
foregoing covenant, the Company may impose stop-transfer instructions with
respect to my Shares until the end of such period.

                                       Very truly yours,

                                       ----------------------------------
                                       Arthur H. Reidel