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                                                                   Exhibit 10.21

                    PROMISSORY NOTE SECURED BY DEED OF TRUST

$150,000.00                                                Palo Alto, California
                                                               December 20, 2001

FOR VALUE RECEIVED, the undersigned, Kenneth Cundy ("Borrower"), promise to pay
to XenoPort, Inc., a Delaware corporation (the "Company"), or order, the
principal amount of One Hundred Fifty Thousand Dollars ($150,000.00). The
outstanding principal amount shall bear interest from such date until paid at a
rate of 4.13% per annum. All payments shall be credited first to accrued
interest and then to principal.

The outstanding principal amount and any interest accrued thereon shall be due
and payable to the holder hereof at 2631 Hanover Street, Palo Alto, California
94304, or such other place as the holder hereof may designate, upon the earlier
of the following dates (collectively, "Maturity Events"):

      (i)   ninety (90) days following the date of termination of the employment
            of Borrower with the Company, whether voluntary or involuntary, and
            whether with cause or without cause;

      (ii)  the date of any sale, conveyance, assignment, alienation or any
            other form of transfer, whether voluntary or involuntary, of that
            certain real property commonly known as 45 Summit Ridge Place,
            Redwood City, CA 94062 (the "Property"), or any part thereof or
            interest therein; except that the following transfers of the
            Property shall not be deemed to be a Maturity Event:

            a.    a transfer upon the death of Borrower to Borrower's surviving
                  spouse (provided the surviving spouse is an obligor hereunder)
                  or to Borrower upon the death of Borrower's surviving spouse;

            b.    a transfer by an obligor hereof whereby such obligor's spouse
                  becomes a co-owner of the Property;

            c.    a transfer resulting from a decree of dissolution of the
                  marriage or legal separation of Borrower and Borrower's spouse
                  or from a property settlement agreement incidental to such a
                  decree which requires the obligor spouse to assume
                  responsibility for the obligations under this Note and the
                  Deed of Trust (hereinafter defined) and pursuant to which
                  Borrower or Borrower's spouse (whoever is the obligor) becomes
                  the sole owner of the Property; or

            d.    a transfer by an obligor hereof into an inter vivos trust in
                  which an obligor hereof is a beneficiary; or

      (iii) the seventh (7) anniversary of the date of this Note.

In the event that any of the following occurs, then unless otherwise prohibited
by law, the holder hereof shall have the option, without demand or notice, to
declare the entire outstanding principal balance of this Note, together with all
accrued and unpaid interest thereon to be immediately due and payable:

      (i)   Borrower's default in the payment of principal or interest when due
            pursuant to the terms hereof;

      (ii)  Borrower's default in its performance of any obligation contained in
            the deed of trust encumbering the Property and securing this Note
            (the "Deed of Trust") or any other deed of trust, security agreement
            or other agreement (including any amendment, modification

                                       1.
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            or extension thereof) which may hereafter be executed by Borrower
            for the purpose of securing this Note;

      (iii) any representation or warranty contained in this Note, the Deed of
            Trust, or any other agreement or instrument executed in connection
            with the loan proves to have been false or misleading in any
            material respect;

      (iv)  Borrower's default in its obligation to pay any indebtedness or to
            perform any other obligation which is secured by a deed of trust or
            other lien on the Property or default under any deed of trust
            securing such indebtedness;

      (v)   Borrower's default in its obligation to pay any indebtedness
            evidenced by any promissory note executed by Borrower and payable to
            the holder hereof or there occurs any other default under any deed
            of trust, mortgage or other document securing repayment of such
            indebtedness; or

      (vi)  the amount of the indebtedness encumbering the Property secured by a
            deed of trust, lien or other encumbrance that is senior to the Deed
            of Trust is increased over the amount of such indebtedness existing
            as of the date of this Note.

The principal amount evidenced by this Note shall be used by Borrower to
purchase the Property which shall become the primary residence of Borrower. This
Note shall be secured by the Deed of Trust given by Borrower to the Company.

In the event any amount owed by Borrower pursuant to this Note is not paid when
due, such unpaid amount shall bear interest from the due date until paid at a
rate equal to the lower of: (i) ten percent (10%) per annum; or (ii) the maximum
rate permitted by law. After such due date, all payments shall be credited first
to accrued interest and then to principal.

If an action is instituted for collection of this Note, the Borrower agrees to
pay court costs and reasonable attorneys' fees incurred by the holder thereof.

Notwithstanding anything to the contrary contained in this Note, so long as
Borrower remains employed with the Company, on each anniversary of the date of
this Note, the Company shall automatically forgive all interest then accrued
pursuant to the terms of this Note; provided that in the event that the
employment of Borrower with the Company terminates prior to the forgiveness of
all of the interest obligations under this Note, the unforgiven, unpaid interest
then accrued hereunder shall become due and payable according to the provisions
above with respect to Maturity Events.

This Note may be amended or modified, and provisions hereof may be waived, only
by the written agreement of Borrower and the holder hereof. No delay or failure
by the holder hereof in exercising any right, power or remedy hereunder shall
operate as a waiver of such right, power or remedy, and a waiver of any right,
power or remedy on any one occasion shall not operate as a bar or waiver of any
such right, power or remedy on any other occasion. Without limiting the
generality of the foregoing, the delay or failure by the holder hereof for any
period of time to enforce collection of any amounts due hereunder shall not be
deemed to be a waiver of any rights of the holder hereof under contract or under
law. The rights of the Company under this Note are in addition to any other
rights and remedies which the holder hereof may have.

                                       2.
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This Note shall be governed by and construed in accordance with the laws of the
State of California.

This Note may be prepaid at any time without penalty.

THIS NOTE, THE DEED OF TRUST AND ALL RELATED DOCUMENTATION ARE EXECUTED
VOLUNTARILY AND WITHOUT ANY DURESS OR UNDUE INFLUENCE ON THE PART OR BEHALF OF
THE PARTIES HERETO, WITH THE FULL INTENT OF CREATING THE OBLIGATIONS AND
SECURITY INTERESTS DESCRIBED HEREIN AND THEREIN. THE PARTIES ACKNOWLEDGE THAT:
(a) THEY HAVE READ SUCH DOCUMENTATION; (b) THEY HAVE BEEN REPRESENTED IN THE
PREPARATION, NEGOTIATION AND EXECUTION OF SUCH DOCUMENTATION BY LEGAL COUNSEL OF
THEIR OWN CHOICE OR THAT THEY HAVE VOLUNTARILY DECLINED TO SEEK SUCH COUNSEL;
(c) THEY UNDERSTAND THE TERMS AND CONSEQUENCES OF THIS NOTE, THE DEED OF TRUST
AND ALL RELATED DOCUMENTATION AND THE OBLIGATIONS THEY CREATE; AND (d) THEY ARE
FULLY AWARE OF THE LEGAL AND BINDING EFFECT OF THIS NOTE, THE DEED OF TRUST AND
THE OTHER DOCUMENTS CONTEMPLATED BY THIS AGREEMENT. WITHOUT LIMITING THE
GENERALITY OF THE FOREGOING, BORROWER HEREBY ACKNOWLEDGES THAT THE COMPANY HAS
MADE NO REPRESENTATION OR WARRANTY TO BORROWER CONCERNING THE INCOME TAX
CONSEQUENCES OF THE LOAN TO BORROWER, AND BORROWER SHALL BE SOLELY RESPONSIBLE
FOR ASCERTAINING AND BEARING SUCH TAX CONSEQUENCES. BORROWER FURTHER
ACKNOWLEDGES THAT (i) THE COMPANY MAY, IN ITS SOLE DISCRETION, DETERMINE THAT IT
IS REQUIRED UNDER THE INTERNAL REVENUE CODE OF 1986, AS AMENDED (THE "CODE"),
AND THE RULES AND REGULATIONS PROMULGATED BY THE INTERNAL REVENUE SERVICE
("IRS") THEREUNDER, TO IMPUTE INTEREST ON THE PRINCIPAL OF THIS NOTE AT THE RATE
SET BY THE IRS, (ii) THE AMOUNT OF ANY SUCH IMPUTED INTEREST WOULD BE DEEMED TO
BE COMPENSATION INCOME TO EMPLOYEE WHICH WOULD BE SUBJECT TO TAX WITHHOLDING,
AND (iii) IF SO DETERMINED BY THE COMPANY, THE COMPANY WOULD REPORT AND WITHHOLD
THE REQUIRED AMOUNT OUT OF THE CURRENT COMPENSATION PAID TO EMPLOYEE IN
ACCORDANCE WITH THE CODE AND THE RULES AND REGULATIONS PROMULGATED THEREUNDER.

BORROWER:                               COMPANY:

Signature: /s/ Kenneth Cundy            Signature: /s/ Ronald W. Barrett
           --------------------                    -----------------------------
              Kenneth Cundy                                 Ronald W. Barrett
                                                            President & CEO

THIS NOTE IS SUBJECT TO SECTION 2924i OF THE CALIFORNIA CIVIL CODE, WHICH
PROVIDES THAT THE HOLDER OF THIS NOTE SHALL GIVE WRITTEN NOTICE TO THE TRUSTOR,
OR HIS SUCCESSOR IN INTEREST, OF PRESCRIBED INFORMATION AT LEAST 90 AND NOT MORE
THAN 150 DAYS BEFORE ANY BALLOON PAYMENT IS DUE.

                                       3.