<PAGE>

                                                                   EXHIBIT 10.18

                               PETOPIA.COM, INC.

           AMENDMENT TO SERIES D PREFERRED STOCK PURCHASE AGREEMENT
           --------------------------------------------------------

     This Amendment to Series D Preferred Stock Purchase Agreement (the
"Amendment") is made as of January 21, 2000 by and between Petopia.com, Inc., a
 ---------
Delaware corporation (the "Company"), a majority of the Purchasers listed on
                           -------
Exhibit A to the Series D Preferred Stock Purchase Agreement dated as of
---------
November 29, 1999 (the "Purchase Agreement") and the purchasers listed on
                        ------------------
Exhibit A attached hereto (the "Additional Purchasers").
---------                       ---------------------

     WHEREAS, the Company and the "Purchasers" (as defined in the Purchase
Agreement) are parties to the Purchase Agreement pursuant to which each
Purchaser purchased shares of Series D Preferred Stock from the Company on the
terms and conditions set forth therein;

     WHEREAS, the Company and a majority of the Purchasers wish to amend the
Purchase Agreement in accordance with Section 7.10 of the Purchase Agreement
(terms defined in the Purchase Agreement are used herein as therein defined);

     WHEREAS, the Additional Purchasers desire to purchase and the Company
desires to sell up to 3,567,542 shares of the Company's Series D Preferred Stock
at the Subsequent Closing (as defined below);

     NOW, THEREFORE, in consideration of the foregoing recitals (which are
hereby incorporated into and shall be deemed part of this Amendment) and of the
covenants and mutual agreements contained in this Amendment, each of the
Company, a majority of the Purchasers and the Additional Purchasers agrees as
follows:

     Section 1.  Amendments.  The Company and a majority of the Purchasers
                 ----------
hereby agree that the Purchase Agreement shall be amended as follows:

     (A)  Section 1.1(a) of the Purchase Agreement shall be amended by (i)
          --------------
replacing "Closing" with "First Closing", (ii) adding "Fourth" before "Restated
                                                       ------          --------
Certificate", and (iii) adding the following at the end of the subsection:
-----------

               "The Company shall also adopt and file with the Secretary of
          State of the State of Delaware on or before the Subsequent Closing (as
          defined below) the Fifth Amended and Restated Certificate of
          Incorporation in the form attached hereto as Exhibit B-1 (the "Fifth
                                                       -----------       -----
          Restated Certificate" and together with the Fourth Restated
          --------------------
          Certificate, the "Restated Certificate")."
                            --------------------

     (B)  Section 1.1(b) of the Purchase Agreement shall be deleted in its
          --------------
entirety and substituting the following therefor:

               "(b)  Subject to the terms and conditions of this Agreement, the
          Purchasers agree to purchase and the Company agrees to sell and issue
          to each Purchaser at the First and Subsequent Closings that number of
          shares of Series D Preferred Stock set forth opposite each such
          Purchaser's name on Exhibit A
                              ---------
<PAGE>

          attached hereto at a purchase price of $5.6061 per share (the
          "Purchase Price"), provided that the purchase price shall be payable
           --------------    --------
          by National Broadcasting Company, Inc. ("NBC") in consideration of the
                                                   ---
          execution and delivery by NBC of the Letter Agreement dated as of the
          date hereof regarding the granting by NBC of advertising credits to
          the Company (the "Letter Agreement"). The shares of Series D Preferred
                            ----------------
          Stock issued to the Purchasers pursuant to this Agreement shall be
          hereinafter referred to as the "Stock.""
                                          -----
     (C)  Section 1.2 of the Purchase Agreement shall be deleted in its
          -----------
entirety and substituting the following therefor:

          "1.2  Closing; Delivery
                -----------------

                (a) The purchase and sale of the Stock shall take place at the
          offices of Perkins Coie LLP, 135 Commonwealth Drive, Suite 250, Menlo
          Park, California, at 10:00 a.m., on November 29, 1999, or at such
          other time and place as the Company and the Purchasers of not less
          than 6,243,200 shares of the Stock mutually agree upon, orally or in
          writing (which time and place are designated as the "First Closing").
                                                               -------------
          The closings of the purchase and sale of up to 9,837,804 shares of
          Stock in the aggregate (not including shares of Stock sold in the
          First Closing, the "Additional Shares") hereunder (the "Subsequent
                              -----------------                   ----------
          Closings" and together with the First Closing, the "Closing") shall be
          --------                                            -------
          held at the offices of Perkins Coie LLP on the date hereof.  Any
          Additional Shares may be sold only to the investors listed on Exhibit
                                                                        -------
          A-1.
          ---

                (b) Subject to the terms of this Agreement, at the Closing, the
          Company shall deliver to each Purchaser a certificate representing the
          Stock being purchased by such Purchaser against payment of the
          purchase price therefor by check payable to the Company, by wire
          transfer to the Company's bank account, by cancellation of
          indebtedness, or by any combination thereof.  At the Subsequent
          Closings, if any, a Supplemental Schedule of Purchasers shall be added
          to this Agreement as Exhibit A-2 and the Company will deliver to each
                               -----------
          additional Purchaser who shall have executed this Agreement (each an

          "Additional Purchaser") a certificate or certificates representing the
          ---------------------
          number of Additional Shares being purchased hereby against (i) payment
          of the purchase price therefor by check payable to the Company, by
          wire transfer to the Company's bank account, by cancellation of
          indebtedness, or by any combination thereof, (ii) in the case of NBC,
          by the execution and delivery of the Letter Agreement and (iii)
          delivery of signature pages to this Agreement and each of the Related
          Agreements (as defined below).  In addition, at any Subsequent Closing
          pursuant to which the Company shall have sold more than 8,918,856
          shares of Stock, the Company shall issue and grant to each Purchaser
          and Additional Purchaser a Series D Warrant, in accordance with the
          terms and conditions of Section 6.3 hereof."

     (D)  The following shall be added to the Purchase Agreement as Section
2.2.5:

                                      -2-
<PAGE>

          "2.2.5  Capitalization.  Immediately prior to the Subsequent Closing,
                  --------------
          the authorized capital of the Company consists of:

                  (a) 49,180,000 shares of Preferred Stock, (i) 11,555,000 of
          which shares have been designated Series A Preferred Stock, 9,755,000
          of which are issued and outstanding and 1,800,000 of which have been
          reserved for issuance upon exercise of outstanding warrants to
          purchase shares of Series A Preferred Stock (the "Series A Warrants"),
                                                            -----------------
          (ii) 8,000,000 of which have been designated Series B Preferred Stock,
          7,736,345 of which are issued and outstanding, (iii) 12,940,620 of
          which have been designated Series C Preferred Stock, 3,017,175 of
          which are issued and outstanding, 4,803,458 of which have been
          reserved for issuance to PETCO Animal Supplies, Inc. ("PETCO") upon
                                                                 -----
          the achievement of certain milestones (the "PETCO Shares"), and
                                                      ------------
          5,119,987 of which have been reserved for issuance upon exercise of
          outstanding warrants to purchase shares of Series C Preferred Stock
          (the "Series C Warrants"), (iv) 14,000,000 of which have been
                -----------------
          designated Series D Preferred Stock, 6,270,262 of which are issued and
          outstanding and 918,948 of which have been reserved for issuance upon
          exercise of warrants issued or issuable to the Purchasers pursuant to
          Section 6.3 herein (the "Series D Warrants"), and (v) 2,330,000 of
                                   -----------------
          which shares have been designated Series E Preferred Stock, 2,007,637
          of which are issued and outstanding, 76,617 of which have been
          reserved for issuance upon exercise of warrants to purchase shares of
          Series E Preferred Stock (the "ICOD Warrants"), and 180,000 of which
                                         -------------
          have been reserved for issuance upon exercise of warrants issued or
          issuable to Greyrock Capital, a division of Banc of America Commercial
          Finance Corporation (the "Greyrock Warrants" and, together with the
                                    -----------------
          ICOD Warrants, the "Series E Warrants").  The rights, privileges and
                              -----------------
          preferences of the Series A, Series B, Series C, Series D and Series E
          Preferred Stock are as stated in the Restated Certificate.  All
          issuances of shares of Series A, Series B, Series C, Series D and
          Series E Preferred Stock referenced above, other than the PETCO
          Shares, have been consummated and, except as described above, the
          Company will not issue additional shares of Series A, Series B, Series
          C, Series D or Series E Preferred Stock.  The shares of Series A,
          Series B, Series C, Series D and Series E Preferred Stock issued by
          the Company were duly authorized, validly issued, fully paid,
          nonassessable, free and clear of any liens or encumbrances (other than
          those imposed by the purchasers thereof), and based in part on the
          representations made by the purchasers thereof, were exempt from
          registration under Section 5 of the Securities Act of 1933, as amended
          and the California Corporate Securities Law of 1968, as amended.  The
          rights of PETCO as set forth in Section 6.6 of the Series C Preferred
          Stock Purchase Agreement dated July 12, 1999 by and between the
          Company and PETCO expired prior to any exercise thereof.

                  (b) 71,180,000 shares of Common Stock, 9,873,718 shares of
          which are issued and outstanding. All of the outstanding shares of
          Common Stock have been duly authorized and validly issued, are fully
          paid and nonassessable, and were issued in compliance with all
          applicable federal and state securities laws.

                                      -3-
<PAGE>

                  (c) The Company has reserved 7,500,000 shares of Common Stock
          for issuance to officers, directors, employees and consultants of the
          Company pursuant to its 1999 Stock Plan duly adopted by the Board of
          Directors and approved by the Company's stockholders (the "Stock
                                                                     -----
          Plan"), of which options to purchase 5,016,769 shares of Common Stock
          ----
          are currently outstanding, 2,024,157 shares of Common Stock have been
          issued and 459,074 shares are available for future issuance under the
          Stock Plan.  The Company has reserved (i) 11,555,000 shares of Common
          Stock for issuance upon conversion of the Series A Preferred Stock,
          (ii) 8,000,000 shares of Common Stock for issuance upon conversion of
          the Series B Preferred Stock, (iii) 12,940,620 shares of Common Stock
          for issuance upon conversion of the Series C Preferred Stock, (iv)
          14,000,000 shares of Common Stock for issuance upon conversion of the
          Series D Preferred Stock, (v) 2,330,000 shares of Common Stock for
          issuance upon conversion of the Series E Preferred Stock, (vi) 585,000
          shares of Common Stock for issuance upon the exercise of outstanding
          warrants to purchase Common Stock issued or issuable to the founders
          of Loveland Pet Products, Inc. (the "Loveland Warrants") and (vii)
                                               -----------------
          917,749 shares of Common Stock for issuance upon the exercise of
          warrants to purchase Common Stock issued or issuable to NBC (the "NBC
                                                                            ---
          Warrants").
          --------

                  (d) Except for (i) the conversion privileges of the Series A
          Preferred Stock, Series B Preferred Stock, Series C Preferred Stock,
          Series D Preferred Stock, Series E Preferred Stock, Series A Warrants,
          Loveland Warrants, the Petco Shares, the Series C Warrants, the Series
          D Warrants, the Series E Warrants and the NBC Warrants, (ii) the
          rights of first refusal as set forth in the Investors' Rights
          Agreement and the Co-Sale Agreement (each as defined below), (iii) the
          rights to participate in the initial public offering as indicated in
          Section 2.6 of the Investors' Rights Agreement (as defined below) and
          in the IPO Allocation Agreement dated as of May 4, 1999 between the
          Company and certain purchasers of Series A Preferred Stock (the "May
                                                                           ---
          IPO Allocation Agreement"), and (iv) 7,500,000 shares of Common Stock
          ------------------------
          reserved for issuance pursuant to the Stock Plan, there are, to our
          knowledge, no options, warrants, conversion privileges or other rights
          (or agreements for any such rights) outstanding to purchase or
          otherwise obtain from the Company any of the Company's securities
          except as set forth on the Schedule of Exceptions."

     (E)  Section 3.2 of the Purchase Agreement shall be amended to delete the
          -----------
last sentence thereof.

     (F)  Section 6.3 of the Purchase Agreement shall be deleted in its
          -----------
entirety and substituting the following therefor:

                  "6.3  Series D Warrants.  If the Company sells more than
                     -----------------
          8,918,856 shares of Stock (the number of any such shares, the "Excess
                                                                         ------
          Shares"), then it will issue to each Purchaser and Additional
          ------
          Purchaser warrants (the "Series D Warrants") to purchase additional
                                   -----------------
          shares of Stock, the exact aggregate number of which shall be equal to
          the product of (x) a fraction, the numerator of which is the

                                      -4-
<PAGE>

          shares of Stock held by such Purchaser or Additional Purchaser (other
          than Excess Shares) and the denominator of which is 8,918,856, and (y)
          the aggregate number of Excess Shares.

               The Series D Warrants, if any, shall be in the form attached
          hereto as Exhibit J and shall terminate five years after the issuance
          thereof, unless earlier exercised.  The exercise price for the Series
          D Warrants shall be equal to the Purchase Price hereunder.  The
          holders of the share of Stock issued pursuant to the exercise of the
          Series D Warrants shall become parties to the Related Agreements upon
          execution and delivery of the signature pages thereto."

     (G)  Section 7.15 of the Purchase Agreement shall be amended by adding the
          ------------
following at the end of such Section:

               "Notwithstanding the foregoing, confidential information shall
          not include any information that is: (i) in the public domain; (ii)
          was already known to the receiving party or within the receiving
          party's possession at the time of receipt thereof from the Company;
          (iii) received by receiving party from a third party without
          reasonable basis for receiving party to believe disclosure was made in
          violation of a confidential agreement with the Company; (iv) approved
          for disclosure by written authorization of the Company; (v)
          independently developed by the receiving party, (vi) published or
          otherwise made available to the public at the time of its receipt by
          receiving party or subsequently became published or available to the
          public other than by a breach of this Agreement; and/or (vii) required
          to be disclosed by oral questions, interrogatories, requests for
          information or documents, subpoena, civil investigative demand, or
          similar process."

     (H)  Exhibit A-1 of the Purchase Agreement shall be amended to add the
          -----------
following entity to the Schedule of Permitted Investors:

          ValueVision International Inc.

     Section 2.  Additional Purchasers.  Each of the Additional Purchasers
                 ---------------------
listed on the attached Exhibit A shall become parties to the Purchase Agreement
                       ---------
by executing this Amendment and by virtue of their inclusion on Exhibit A
                                                                ---------
attached hereto, which Exhibit A shall be added to the Purchase Agreement as
                       ---------
Exhibit A-2.
-----------

     Section 3.  Fifth Restated Certificate.  Exhibit B attached hereto shall be
                 --------------------------   ---------
added to the Purchase Agreement as Exhibit B-1.
                                   -----------

     Section 4.  Series D Warrants.  Exhibit C attached hereto shall be added to
                 -----------------   ---------
the Purchase Agreement as Exhibit J.  The Series D Warrants to be issued
                          ---------
pursuant to Section 6.3 of the Purchase Agreement, as amended by this Amendment,
will be issued to the Purchasers and Additional Purchasers listed on Exhibit D
                                                                     ---------
attached hereto in the amounts set for opposite their names, which Exhibit D
                                                                   ---------
shall be added to the Purchase Agreement as Exhibit K.
                                            ---------

                                      -5-
<PAGE>

     Section 5.  Governing Law.  This Amendment and all acts and transactions
                 -------------
pursuant hereto and the rights and obligations of the parties hereto shall be
governed, construed and interpreted in accordance with the laws of the State of
California, without giving effect to principles of conflicts of laws.

     Section 6.  Amendments.  This Amendment may not be modified or amended
                 ----------
except by written agreement signed by the Company and a majority of the holders
of the Series D Preferred Stock.

     Section 7.  Counterparts.  This Amendment may be executed in two or more
                 ------------
counterparts, each of which shall be deemed an original and all of which
together shall constitute one instrument.

                 [Remainder of page intentionally left blank]

                                      -6-
<PAGE>

     IN WITNESS WHEREOF, a majority of the Purchasers, the Additional Purchasers
and the Company have caused this Amendment to be duly executed as of the date
first above written.

                                        COMPANY:

                                        PETOPIA.COM, INC.


                                        By: /s/ Andrea C. Reisman
                                            ----------------------------
                                            Name:  Andrea C. Reisman
                                            Title: Chief Executive Officer


                                        PURCHASERS:

<PAGE>

                                TCV III (GP)
                                a Delaware General Partnership
                                By:  Technology Crossover Management III, L.L.C.
                                Its: General Partner

                                By: /s/ Robert C. Bensky
                                    --------------------------------------------
                                    Name:  Robert C. Bensky
                                    Title: Chief Financial Officer


                                TCV III, L.P.
                                a Delaware Limited Partnership
                                By:  Technology Crossover Management III, L.L.C.
                                Its: General Partner

                                By: /s/ Robert C. Bensky
                                    --------------------------------------------
                                    Name:  Robert C. Bensky
                                    Title: Chief Financial Officer


                                TCV III (Q), L.P.
                                a Delaware Limited Partnership
                                By:  Technology Crossover Management III, L.L.C.
                                Its: General Partner

                                By: /s/ Robert C. Bensky
                                    --------------------------------------------
                                    Name:  Robert C. Bensky
                                    Title: Chief Financial Officer


                                TCV III STRATEGIC PARTNERS, L.P.
                                a Delaware Limited Partnership
                                By:  Technology Crossover Management III, L.L.C.
                                Its: General Partner

                                By: /s/ Robert C. Bensky
                                    --------------------------------------------
                                    Name:  Robert C. Bensky
                                    Title: Chief Financial Officer


<PAGE>

                                Mailing Address:
                                    Technology Crossover Ventures
                                    56 Main Street, Suite 210
                                    Millburn, NJ 07041
                                    Attention:  Robert C. Bensky
                                    Phone: (973) 467-5320
                                    Fax:   (973) 467-5323

                                with a copy to:
                                    Technology Crossover Ventures
                                    575 High Street, Suite 400
                                    Palo Alto, CA 94301
                                    Attention: Jay C. Hoag
                                    Phone: (650) 614-8210
                                    Fax:   (650) 614-8222

<PAGE>

                             PETCO ANIMAL SUPPLIES, INC.

                             By: /s/ Brian K. Devine
                                 ------------------------------------------
                                 Name:  Brian K. Devine
                                 Title: Chairman, President and Chief Executive
                                        Officer


                             Mailing Address:
                                 9125 Rehco Road
                                 San Diego, CA 92121

<PAGE>

                             ARKARO HOLDING, B.V.

                             By: /s/ Maria C. van der Sluijs-Plantz
                                 -----------------------------------------------
                                 Name:  Maria C. van der Sluijs-Plantz
                                 Title: Managing Director


                             Mailing Address:
                                 Locatellikade 1
                                 Parnassustoren
                                 1076 AZ Amsterdam
                                 P.O. Box 75215
                                 1070 AE Amsterdam
                                 The Netherlands
                                 Attention: Maria C. van der Sluijs-Plantz

<PAGE>

                         ADDITIONAL PURCHASERS:


                         NATIONAL BROADCASTING COMPANY, INC.
                         a Delaware corporation

                         By: __________________________________________________
                             Name:
                             Title:


                         Mailing Address:
                             30 Rockefeller Plaza
                             New York, NY 10122
                             Attn: President, NBC Interactive Media
                             With a copy to: VP, Law, Corporate Transactions


                         VALUE VISION INTERNATIONAL INC.
                         a Minnesota corporation

                         By: ________________________________________________
                             Name:
                             Title:


                         Mailing Address:
                             6740 Shady Oak Road
                             Eden Prairie, MN 55344
                             Phone: (612) 947-5200
                             Fax:   (612) 947-0188

                         with a copy to:
                             Faegre & Benson LLP
                             2200 Norwest Center
                             90 South Seventh Street
                             Minneapolis, MN 55402
                             Attn:  Steven C. Kennedy
                             Phone: (612) 336-3600
                             Fax:   (612) 336-2600