SEPARATION AGREEMENT AND RELEASE

Exhibit 10.1

SEPARATION AGREEMENT AND RELEASE

RECITALS

This Separation Agreement and Release (“Agreement”) is made by and between Mark Robillard (Employee”) and Pharsight Corporation (“Company”) (jointly referred to as the “Parties”):

WHEREAS, Employee was employed by the Company;

WHEREAS, the Company and Employee entered into a Proprietary Information and Inventions Agreement (the “Confidentiality Agreement”);

WHEREAS, Employee’s employment was terminated on January 15, 2005 (the “Termination Date”);

WHEREAS, the Company and Employee have entered into stock option agreements dated October 26, 2001, April 24, 2003 and April 21, 2004 granting the Employee the option to purchase shares of the Company’s common stock subject to the terms and conditions of the Company’s 2000 Equity Incentive Plan, as amended, and the Stock Option Agreement (the “Stock Option Agreements”); and

WHEREAS, the Company and Employee have entered into the Indemnity Agreement dated March 31, 2003 (“Indemnity Agreement”).

WHEREAS, the Parties wish to resolve any and all disputes, claims, complaints, grievances, charges, actions, petitions and demands that the Employee may have against the Company, including, but not limited to, any and all claims arising or in any way related to Employee’s employment with or separation from the Company;

NOW THEREFORE, in consideration of the promises made herein, the Parties hereby agree as follows:

COVENANTS

1. Consideration . The Company agrees to pay Employee the equivalent of his/her base salary, less applicable withholding, for a period of six (6) months from the Termination Date. The first payment will be made on the first regular payroll date following the Effective Date of this Agreement and will continue, thereafter, in accordance with the Company’s regular payroll practices, for six (6) months (the “Payment Period”). During the Payment Period, Employee will not be entitled to accrual of any employee benefits, including, but not limited to, vesting in stock options or vacation benefits.

2. Stock . The Parties agree that for purposes of determining the number of shares of the Company’s common stock which Employee is entitled to purchase from the Company, pursuant to the exercise of outstanding options, the Employee will be considered to have vested only up to the Termination Date. Employee acknowledges that as of the Termination Date, he will have vested in 139,583 options and no more. The exercise of any stock options shall continue to be subject to the terms and conditions of the Stock Option Agreements.

3. Benefits . Employee’s health insurance benefits will continue for a period of six (6) months, and will cease on July 31, 2005, subject to Employee’s right to continue his health insurance under COBRA. Employee’s participation in all other benefits and incidents of employment ceased on the Termination Date. Employee ceased accruing employee benefits, including, but not limited to, vacation time and paid time off, as of the Termination Date.

4. Confidential Information . Employee shall continue to maintain the confidentiality of all confidential and proprietary information of the Company and shall continue to comply with the terms and conditions of the Confidentiality Agreement between Employee and the Company. Employee shall return all of the Company’s property and confidential and proprietary information in his possession to the Company. By signing this Agreement, Employee represents and declares under penalty of perjury under the laws of the State of California that he has returned all Company property.

5. Payment of Salary . Employee acknowledges and represents that the Company has paid all salary, wages, bonuses, accrued vacation, commissions and any and all other benefits due to Employee.

6. Release of Claims . Employee agrees that the foregoing consideration represents settlement in full of all outstanding obligations owed to Employee by the Company and its officers, managers, supervisors, agents and employees. Employee, on his own behalf, and on behalf of his/her respective heirs, family members, executors, agents, and assigns, hereby fully and forever releases the Company and its officers, directors, employees, agents, investors, shareholders, administrators, affiliates, divisions, subsidiaries, predecessor and successor corporations, and assigns (the “Releasees”), from, and agree not to sue concerning, any claim, duty, obligation or cause of action relating to any matters of any kind, whether presently known or unknown, suspected or unsuspected, that Employee may possess arising from any omissions, acts or facts that have occurred up until and including the Effective Date of this Agreement including, without limitation:

(a) any and all claims relating to or arising from Employee’s employment relationship with the Company and the termination of that relationship;

(b) any and all claims relating to, or arising from, Employee’s right to purchase, or actual purchase of shares of stock of the Company, including, without limitation, any claims for fraud, misrepresentation, breach of fiduciary duty, breach of duty under applicable state corporate law, and securities fraud under any state or federal law;

(c) any and all claims under the law of any jurisdiction including, but not limited to, wrongful discharge of employment; constructive discharge from employment; termination in violation of public policy; discrimination; breach of contract, both express and implied; breach of a covenant of good faith and fair dealing, both express and implied; promissory estoppel; negligent or intentional infliction of emotional distress; negligent or intentional misrepresentation; negligent or intentional interference with contract or prospective economic advantage; unfair business practices; defamation; libel; slander; negligence; personal injury; assault; battery; invasion of privacy; false imprisonment; and conversion;

(d) any and all claims for violation of any federal, state or municipal statute, including, but not limited to, Title VII of the Civil Rights Act of 1964, the Civil Rights Act of 1991, the Age Discrimination in Employment Act of 1967, the Americans with Disabilities Act of 1990, the Fair Labor Standards Act, the Employee Retirement Income Security Act of 1974, The Worker Adjustment and Retraining Notification Act, the Older Workers Benefit Protection Act; the Family and Medical Leave Act; the California Family Rights Act; the California Fair Employment and Housing Act, and the California Labor Code, including, but not limited to Labor Code sections 1400-1408;

(e) any and all claims for violation of the federal, or any state, constitution;

(f) any and all claims arising out of any other laws and regulations relating to employment or employment discrimination;

(g) any claim for any loss, cost, damage, or expense arising out of any dispute over the non-withholding or other tax treatment of any of the proceeds received by Employee as a result of this Agreement; and

(h) any and all claims for attorneys’ fees and costs.

The Company and Employee agree that the release set forth in this section shall be and remain in effect in all respects as a complete general release as to the matters released. This release does not extend to any obligations incurred under this Agreement.

7. Acknowledgement of Waiver of Claims Under ADEA . Employee acknowledges that he/she is waiving and releasing any rights he/she may have under the Age Discrimination in Employment Act of 1967 (“ADEA”) and that