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                                                                  Exhibit 10.18

                              PHARSIGHT CORPORATION

                             1995 Stock Option Plan

                             As Adopted May 12, 1995
                           As Amended January 26, 1996
                            As Amended April 19, 1996
                    Approved by the Shareholders May 2, 1996

      1. Purpose. This 1995 Stock Option Plan (the "Plan") is established as a
compensatory plan to attract, retain and provide equity incentives to selected
persons to promote the financial success of Pharsight Corporation, a California
corporation (the "Company"). Capitalized terms not previously defined herein are
defined in Section 17 of this Plan.

      2. Types of Options and Shares. Options granted under this Plan (the
"Options") may be either (a) incentive stock options ("ISOs") within the meaning
of Section 422 of the Internal Revenue Code of 1986, as amended (the "Code"), or
(b) nonqualified stock options ("NQSOs"), as designated at the time of grant.
The shares of stock that may be purchased upon exercise of Options granted under
this Plan (the "Shares") are shares of the common stock of the Company.

      3. Number of Shares. The aggregate number of Shares that may be issued
pursuant to Options granted under this Plan is 507,000 Shares, subject to
adjustment as provided in this Plan. If any Option expires or is terminated
without being exercised in whole or in part, the unexercised or released Shares
from such Option shall be available for future grant and purchase under this
Plan. At all times during the term of this Plan, the Company shall reserve and
keep available such number of Shares as shall be required to satisfy the
requirements of outstanding Options under this Plan.

      4. Eligibility. Options may be granted to employees, officers, directors,
consultants, independent contractors and advisers of the Company or any Parent,
Subsidiary or Affiliate of the Company (provided such consultants, independent
contractors and advisers render bona fide services not in connection with the
offer and sale of securities in a capital-raising transaction). ISOs may be
granted only to employees of the Company or of a Parent or Subsidiary of the
Company (including officers and directors who are also employees). The Committee
(as defined in Section 14) in its sole discretion shall select the recipients of
Options ("Optionees"). An Optionee may be granted more than one Option under
this Plan. The Company may also, from time to time, assume outstanding options
granted by another company, whether in connection with an acquisition of such
other company or otherwise, by either (i) granting an Option under this Plan in
replacement of the option assumed by the Company, or (ii) treating the assumed
option as if it had been granted under this Plan if the terms of such assumed
option could be applied to an Option granted under this Plan. Such assumption
shall be permissible if the holder of the assumed option would have been
eligible to be granted an Option under this Plan if the other company had
applied the rules of this Plan to such grant.

      5. Terms and Conditions of Options. The Committee shall determine whether
each Option is to be an ISO or an NQSO, the number of Shares subject to the
Option, the exercise price of the Option, the period during which the Option may
be exercised, and all other terms and conditions of the Option, subject to the
following:


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            (a) Form of Option Grant. Each Option granted under this Plan shall
be evidenced by a written Stock Option Grant (the "Grant") in such form (which
need not be the same for each Optionee) as the Committee shall from time to time
approve, which Grant shall comply with and be subject to the terms and
conditions of this Plan.

            (b) Date of Grant. The date of grant of an Option shall be the date
on which the Committee makes the determination to grant such Option unless
otherwise specified by the Committee. The Grant representing the Option will be
delivered to the Optionee with a copy of this Plan within a reasonable time
after the granting of the Option. The Committee shall also determine the vesting
start date for each Option. Unless otherwise specified by the Committee, the
vesting start date shall be (i) for employees of the Company, the first day of
full-time employment, and (ii) for consultants, independent contractors and
advisors, the date of grant.

            (c) Exercise Price. The exercise price of an Option shall be
determined by the Committee; provided that (i) the exercise price of an NQSO
shall not be less than 85% of the Fair Market Value of the Shares on the date
the Option is granted; (ii) the exercise price of an ISO shall not be less than
100% of the Fair Market Value of the Shares on the date the Option is granted;
and (iii) the exercise price of any Option granted to a person owning more than
10% of the total combined voting power of all classes of stock of the Company or
any Parent or Subsidiary of the Company ("Ten Percent Shareholder") shall not be
less than 110% of the Fair Market Value of the Shares on the date the Option is
granted.

            (d) Exercise Period. Options shall be exercisable within the times
or upon the events determined by the Committee as set forth in the Grant;
provided, however, that no Option shall be exercisable after the expiration of
ten (10) years from the date the Option is granted, and provided further, that
no ISO granted to a Ten Percent Shareholder shall be exercisable after the
expiration of five (5) years from the date the Option is granted, and provided
further, that all Options granted under this Plan must become exerciseable as to
at least twenty percent (20%) of the Shares for each full year from the vesting
start date so long as the Optionee is providing services to the Company.

            (e) Limitations on ISOs. The aggregate Fair Market Value (determined
as of the time an Option is granted) of stock with respect to which ISOs are
exercisable for the first time by an Optionee during any calendar year (under
this Plan or under any other incentive stock option plan of the Company or any
Parent or Subsidiary of the Company) shall not exceed $100,000. If the Fair
Market Value of Shares with respect to which ISOs are exercisable for the first
time by an Optionee during any calendar year exceeds $100,000, the Options for
the first $100,000 worth of Shares to become exercisable in such calendar year
shall be ISOs and the Options for the amount in excess of $100,000 that become
exercisable in that calendar year shall be NQSOs. In the event that the Code or
the regulations promulgated thereunder are amended after the effective date of
this Plan to provide for a different limit on the Fair Market Value of Shares
permitted to be subject to ISOs, such different limit shall be incorporated
herein and shall apply to any Options granted after the effective date of such
amendment.

            (f) Options Non-Transferable. Options granted under this Plan, and
any interest therein, shall not be transferable or assignable by the Optionee,
and may not be made subject to execution, attachment or similar process,
otherwise than by will or by the laws of descent and distribution, and shall be
exercisable during the lifetime of the Optionee only by the Optionee; provided,
however, that NQSOs held by an Optionee who is not an officer or director of the
Company or other person whose transactions in the Company's common stock are
subject to Section 16(b) of the Securities Exchange Act of 1934, as amended (the
"Exchange Act") (in each case, an "Insider"), may be transferred to such family
members, trusts and charitable institutions as the Committee, in its sole
discretion, shall approve at the time of the grant of such Option.


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            (g) Assumed Options. In the event the Company assumes an option
granted by another company, the terms and conditions of such option shall remain
unchanged (except the exercise price and the number and nature of shares
issuable upon exercise, which will be adjusted appropriately pursuant to Section
424(a) of the Code). In the event the Company elects to grant a new option
rather than assuming an existing option (as specified in Section 4), such new
option may be granted with a similarly adjusted exercise price.

      6. Exercise of Options. The following provisions shall apply to the
exercise of Options:

            (a) Notice. Options may be exercised only by delivery to the Company
of a written stock option exercise agreement (the "Exercise Agreement") in a
form approved by the Committee (which need not be the same for each Optionee),
stating the number of Shares being purchased, the restrictions imposed on the
Shares, if any, and such representations and agreements regarding Optionee's
investment intent and access to information, if any, as may be required by the
Company to comply with applicable securities laws, together with payment in full
of the exercise price for the number of Shares being purchased.

            (b) Payment. Payment for the Shares purchased upon exercise of an
Option may be made in cash (by check) or, where approved by the Committee in its
sole discretion at the time of grant and where permitted by law: (i) by
cancellation of indebtedness of the Company to the Optionee; (ii) by surrender
of shares of common stock of the Company (free and clear of all liens, claims,
encumbrances and security interests) having a Fair Market Value equal to the
applicable exercise price of the Options, that (A) have been owned by Optionee
for more than six (6) months and have been paid for within the meaning of
Securities and Exchange Commission ("SEC") Rule 144 and, if such Shares were
purchased from the Company by use of a promissory note, such note has been fully
paid with respect to such shares, or (B) were obtained by Optionee in the open
public market; (iii) by tender of a full recourse promissory note having such
terms as may be approved by the Committee and bearing interest at a rate
sufficient to avoid imputation of income under Sections 483 and 1274 of the
Code; (iv) by waiver of compensation due or accrued to Optionee for services
rendered; (v) provided that a public market for the Company's stock exists,
through a "same day sale" commitment from Optionee and a broker-dealer that is a
member of the National Association of Securities Dealers (an "NASD Dealer")
whereby Optionee irrevocably elects to exercise the Option and to sell a portion
of the Shares so purchased to pay for the exercise price and whereby the NASD
Dealer irrevocably commits upon receipt of such Shares to forward the exercise
price directly to the Company; (vi) provided that a public market for the
Company's stock exists, through a "margin" commitment from Optionee and an NASD
Dealer whereby Optionee irrevocably elects to exercise the Option and to pledge
the Shares so purchased to the NASD Dealer in a margin account as security for a
loan from the NASD Dealer in the amount of the exercise price, and whereby the
NASD Dealer irrevocably commits upon receipt of such Shares to forward the
exercise price directly to the Company; or (vii) by any combination of the
foregoing. Optionees who are not employees of the Company shall not be entitled
to purchase Shares with a promissory note unless the note is adequately secured
by collateral other than the Shares.

            (c) Withholding Taxes. Prior to issuance of the Shares upon exercise
of an Option, the Optionee shall pay or make adequate provision for any federal
or state withholding obligations of the Company, if applicable.

            (d) Limitations on Exercise. Notwithstanding the exercise periods
set forth in the Grant, exercise of an Option shall always be subject to the
following:

                  (i) If the Optionee ceases to be employed by the Company or
any Parent, Subsidiary or Affiliate of the Company for any reason except death
or disability, then the Optionee may exercise such Optionee's Options to the
extent (and only to the extent) that they would have been exercisable upon the
date of termination of Optionee's employment, within three (3) months after the
date of termination (or such shorter time


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period as may be specified in the Grant, so long as such shorter time period
provides that the Optionee may exercise his or her Option for at least 30 days
from the date of termination), but in any event no later than the expiration
date of the Options.

                  (ii) If the Optionee's employment with the Company or any
Parent, Subsidiary or Affiliate of the Company is terminated because of the
death of the Optionee or the disability of the Optionee, the Optionee's Options
may be exercised to the extent (and only to the extent) that they would have
been exercisable by the Optionee on the date of termination of the Optionee's
employment, by the Optionee (or the Optionee's legal representative) within
twelve (12) months after the date of termination of the Optionee's employment
(or such shorter time period as may be specified in the Grant so long as such
shorter time period is at least six (6) months), but in any event no later than
the expiration date of the Options. If the Option is an ISO and if Optionee's
disability does not fall within the meaning of that term in Section 22(e)(3) of
the Code, then such ISO shall automatically become a NQSO if it has not been
exercised prior to the end of three (3) months from the date of termination.

                  (iii) The Committee shall have discretion to determine whether
the Optionee has ceased to be employed by the Company or any Parent, Subsidiary
or Affiliate of the Company and the effective date on which such employment
terminated.

                  (iv) In the case of an Optionee who is a director, consultant,
independent contractor or adviser, the Committee will have the discretion to
determine whether the Optionee is "employed by the Company or any Parent,
Subsidiary or Affiliate of the Company" pursuant to the foregoing Sections.

                  (v) The Committee may specify a reasonable minimum number of
Shares that may be purchased on any exercise of an Option, provided that such
minimum number will not prevent Optionee from exercising the full number of
Shares as to which the Option is then exercisable.

                  (vi) An Option shall not be exercisable unless such exercise
is in compliance with the Securities Act of 1933, as amended (the "Securities
Act"), all applicable state securities laws and the requirements of any stock
exchange or national market system upon which the Shares may then be listed, as
they are in effect on the date of exercise. The Company shall be under no
obligation to register the Shares with the SEC or to effect compliance with the
registration, qualification or listing requirements of any state securities
laws, stock exchange or national market system, and the Company shall have no
liability for any inability or failure to do so.

      7. Restrictions on Shares. At the discretion of the Committee, the Company
may reserve to itself and/or its assignee(s) in the Grant a right of first
refusal to purchase all Shares that an Optionee (or a subsequent transferee) may
propose to transfer to a third party. The Company may also reserve to itself
and/or its assignee(s) in the Grant a right to repurchase a portion of or all
Shares held by an Optionee upon Optionee's termination of employment or service
with the Company or a Parent, Subsidiary or Affiliate of the Company, for any
reason within 90 days of such termination for cash or cancellation of purchase
money indebtedness at the higher of (i) Optionee's original purchase price or
(ii) the Fair Market Value of such Shares on the date of termination.

      8. Modification, Extension, and Renewal of Options. The Committee shall
have the power to modify, extend or renew outstanding Options and to authorize
the grant of new Options in substitution therefor, provided that any such action
may not, without the written consent of the Optionee, impair any rights under
any Option previously granted. Any outstanding ISO that is modified, extended,
renewed or otherwise altered shall be treated in accordance with Section 424(h)
of the Code. The Committee shall have the power to reduce the exercise price of
outstanding Options without the consent of Optionees by a written notice to the
Optionees affected;


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provided, however, that the exercise price per Share may not be reduced below
the minimum exercise price that would be permitted under Section 5(c) of this
Plan for Options granted on the date the action is taken to reduce the exercise
price.

      9. Privileges of Stock Ownership. No Optionee shall have any of the rights
of a shareholder with respect to any Shares subject to an Option until such
Option is properly exercised. No adjustment shall be made for dividends or
distributions or other rights for which the record date is prior to such date of
exercise, except as provided in this Plan. The Company shall provide to each
Optionee a copy of the annual financial statements of the Company within 120
days after the close of each fiscal year of the Company.

      10. No Obligation to Employ. Nothing in this Plan or any Option granted
under this Plan shall confer on any Optionee any right to continue in the employ
of, or other relationship with, the Company or any Parent, Subsidiary or
Affiliate of the Company or limit in any way the right of the Company or any
Parent, Subsidiary or Affiliate of the Company to terminate Optionee's
employment or other relationship at any time, with or without cause.

      11. Adjustment of Option Shares. In the event that the number of
outstanding shares of common stock of the Company is changed by a stock
dividend, stock split, reverse stock split, combination, reclassification or
similar change in the capital structure of the Company without consideration,
then the number of Shares available under this Plan and the number of Shares
subject to outstanding Options and the exercise price per Share of such Options
shall be proportionately adjusted, subject to any required action by the Board
of Directors (the "Board") or shareholders of the Company and compliance with
applicable securities laws; provided, however, that a fractional share shall not
be issued upon exercise of any Option and any fractions of a Share that would
have resulted shall either be cashed out at Fair Market Value or the number of
Shares issuable under the Option shall be rounded up to the nearest whole
number, as determined by the Committee; and provided further that the exercise
price may not be decreased to below the par value, if any, for the Shares.

      12. Assumption of Options by Successor. In the event of (a) a merger or
consolidation in which the Company is not the surviving corporation (other than
a merger or consolidation with a wholly owned subsidiary, a reincorporation of
the Company in a different jurisdiction, or other transaction in which there is
no substantial change in the shareholders of the Company and the Options granted
under this Plan are assumed by the successor corporation), (b) a dissolution or
liquidation of the Company, (c) the sale of all or substantially all of the
assets of the Company, or (d) any other transaction which qualifies as a
"corporate transaction" under Section 424(a) of the Code wherein the
shareholders of the Company give up all of their equity interest in the Company
(except for the acquisition of all or substantially all of the outstanding
shares of the Company), any or all outstanding Options may be assumed by the
successor corporation, which assumption shall be binding on all Optionees. In
the alternative, the successor corporation may substitute an equivalent option
or provide substantially similar consideration to Optionees as was provided to
shareholders (after taking into account the existing provisions of Optionees'
options, such as the exercise price and the vesting schedule). The successor
corporation may also issue, in place of outstanding shares of the Company held
by Optionee as a result of the exercise of an Option that are subject to
repurchase, substantially similar shares or other property subject to similar
repurchase restrictions no less favorable to Optionee. In the event such
successor corporation, if any, refuses to assume or substitute Options, as
provided above, the Options shall expire upon consummation of the transaction.
Subject to the foregoing provisions of this Section 12, in the event of the
occurrence of any transaction described in clauses (a) - (d) of this Section 12,
all outstanding Options shall be treated as provided in the applicable agreement
or plan of merger, consolidation, dissolution, liquidation, sale of assets or
other "corporate transaction".


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      13. Assumption of Options by the Company. The Company, from time to time,
also may substitute or assume outstanding stock options granted by another
company, whether in connection with an acquisition of such other company or
otherwise, by either (a) granting an Option under this Plan in substitution of
such other company's option, or (b) assuming such other company's option as if
it had been granted under this Plan if the terms of such assumed option could be
applied to an Option granted under this Plan. Such substitution or assumption
shall be permissible if the holder of the substituted or assumed Option would
have been eligible to be granted an Option under this Plan if the other company
had applied the rules of this Plan to such grant. In the event the Company
assumes an option granted by another company, the terms and conditions of such
option shall remain unchanged (except that the exercise price and the number and
nature of Shares issuable upon exercise of any such option will be adjusted
appropriately pursuant to Section 424(a) of the Code). In the event the Company
elects to grant a new Option rather than assuming an existing option, such new
Option may be granted with a similarly adjusted exercise price.

      14. Adoption and Shareholder Approval. This Plan shall become effective on
the date that it is adopted by the Board of the Company. This Plan shall be
approved by the shareholders of the Company, in any manner permitted by
applicable corporate law, within twelve months before or after the date this
Plan is adopted by the Board. Upon the effective date of the Plan, the Board may
grant Options pursuant to this Plan; provided that, in the event that
shareholder approval is not obtained within the time period provided herein, all
Options granted hereunder shall terminate. No Option that is issued as a result
of any increase in the number of shares authorized to be issued under this Plan
shall be exercised prior to the time such increase has been approved by the
shareholders of the Company and all such Options granted pursuant to such
increase shall similarly terminate if such shareholder approval is not obtained.
After the Company becomes subject to Section 16(b) of the Exchange Act, the
Company will comply with the requirements of Rule 16b-3 of the Exchange Act
("Rule 16b-3") with respect to shareholder approval.

      15. Administration. This Plan may be administered by the Board or a
committee appointed by the Board (the "Committee"). If, at the time the Company
registers its Common Stock under the Exchange Act, a majority of the Board is
not comprised of Disinterested Persons, the Company will take appropriate steps
to comply with the disinterested director requirements of Section 16(b) of the
Exchange Act, which may consist of the appointment by the Board of a committee
consisting of not less than three persons (who need not be members of the
Board), each of whom is a Disinterested Person. As used in this Plan, references
to the "Committee" shall mean either the committee appointed by the Board to
administer this Plan or the Board if no committee has been established. After
registration of the Company's Common Stock under the Exchange Act, Board members
who are not Disinterested Persons may not vote on any matters affecting the
administration of this Plan or on the grant of any Options pursuant to this Plan
to Insiders, but any such member may be counted for determining the existence of
a quorum at any meeting of the Board during which action is taken with respect
to Options or administration of this Plan and may vote on the grant of any
Options pursuant to this Plan otherwise than to Insiders. The interpretation by
the Committee of any of the provisions of this Plan or any Option granted under
this Plan shall be final and binding upon the Company and all persons having an
interest in any Option or any Shares purchased pursuant to an Option. The
Committee may delegate to officers of the Company the authority to grant Options
under this Plan to Optionees who are not Insiders of the Company.

      16. Term and Termination of Plan; Amendment. Options may be granted
pursuant to this Plan from time to time within a period of ten (10) years from
the date on which this Plan is adopted by the Board. The Committee may at any
time terminate or amend this Plan in any respect including (but not limited to)
amendment of any form of grant, exercise agreement or instrument to be executed
pursuant to this Plan; provided, however, that the Committee shall not, without
the approval of the shareholders of the Company, amend this Plan in any manner
that requires shareholder approval pursuant to the Code or the regulations
promulgated thereunder as such


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provisions apply to ISO plans or pursuant to the Exchange Act or Rule 16b-3 (or
its successor) promulgated thereunder.

      17. Certain Definitions. As used in this Plan, the following terms shall
have the following meanings:

            (a) "Parent" means any corporation (other than the Company) in an
unbroken chain of corporations ending with the Company if, at the time of the
granting of the Option, each of such corporations other than the Company owns
stock possessing 50% or more of the total combined voting power of all classes
of stock in one of the other corporations in such chain.

            (b) "Subsidiary" means any corporation (other than the Company) in
an unbroken chain of corporations beginning with the Company if, at the time of
granting of the Option, each of the corporations other than the last corporation
in the unbroken chain owns stock possessing 50% or more of the total combined
voting power of all classes of stock in one of the other corporations in such
chain.

            (c) "Affiliate" means any corporation that directly, or indirectly
through one or more intermediaries, controls or is controlled by, or is under
common control with, another corporation, where "control" (including the terms
"controlled by" and "under common control with") means the possession, direct or
indirect, of the power to cause the direction of the management and policies of
the corporation, whether through the ownership of voting securities, by contract
or otherwise.

            (d) "Disinterested Person" shall have the meaning set forth in Rule
16b-3(d)(3) as promulgated by the SEC under Section 16(b) of the Exchange Act,
as such rule is amended from time to time and as interpreted by the SEC.

            (e) "Fair Market Value" shall mean the fair market value of the
Shares as determined by the Committee from time to time in good faith. If a
public market exists for the Shares, the Fair Market Value shall be the average
of the last reported bid and asked prices for common stock of the Company on the
last trading day prior to the date of determination (or the average closing
price over the number of consecutive working days preceding the date of
determination as the Committee shall deem appropriate) or, in the event the
common stock of the Company is listed on a stock exchange or on the NASDAQ
National Market System, the Fair Market Value shall be the closing price on such
exchange or quotation system on the last trading day prior to the date of
determination (or the average closing price over the number of consecutive
working days preceding the date of determination as the Committee shall deem
appropriate).


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