<PAGE>
 
                                                                 Exhibit 10.25

         AMENDMENT TO AMENDED AND RESTATED LOAN AND SECURITY AGREEMENT
         -------------------------------------------------------------

     This Amendment to Amended and Restated Loan and Security Agreement (this
"Amendment") is entered into as of January 28, 1999, by and between Silicon
Valley Bank ("Bank") and Inktomi Corporation ("Borrower").

                                  RECITALS
                                  --------

     Borrower and Bank are parties to that certain Amended and Restated Loan and
Security Agreement dated as of September 2, 1998 (the "Agreement").  Borrower
and Bank desire to amend the Agreement in accordance with the terms of this
Amendment.

     NOW, THEREFORE, the parties agree as follows:

     1.  Amendments to Agreement.  The Agreement is hereby amended as follows:
         -----------------------                                              

         (a) The following definitions in Section 1.1 are amended to read as
         follows: 

         "Committed Line" means Six Million Five Hundred Thousand
         Dollars ($6,500,000). 

         "Revolving Maturity Date" means June 30, 1999.

         (b) The following terms are added to Section 1.1 in their proper
         alphabetical order:

          "Equity Issuance" means, as applied to any Person, the sale or
     issuance by such Person of (i) any capital stock of such Person, (ii) any
     options, warrants or other similar rights exercisable in respect of such
     capital stock, or (iii) any other security or instrument representing an
     equity interest (or the right to obtain an equity interest) in such Person.

          "Net Proceeds" means, with respect to any Equity Issuance, the gross
     proceeds received by the issuer from the issuance less all legal and
     accounting expenses, commissions and other fees and expenses incurred or to
     be incurred and all federal, state, local and foreign taxes assessed in
     connection therewith.

          (c)  Section 6.10 is amended in its entirety to read as follows:

          6.10  Tangible Net Worth.  Borrower shall maintain, as of the last day
                ------------------                                              
     of each fiscal quarter, a Tangible Net Worth of not less than $30,000,000
     plus (i) 75% of net income (but not loss) for each fiscal quarter of the
     Borrower commencing with the quarter ending March 30, 1999, plus (ii) 100%
     of the Net Proceeds of any Equity Issuance by the Borrower after the date
     hereof.

          (d)  Exhibit C is replaced with Exhibit C hereto.
               ---------                  ---------        

     2.   Conditions Precedent to Effectiveness.  The effectiveness of this
          -------------------------------------
Amendment is subject to the further conditions precedent that Bank shall have
received a facility fee in the amount of Eight Thousand Three Hundred Eighty
Dollars ($8,380), plus an amount equal to the Bank Expenses incurred in
connection with this Amendment.

     3.   Representation and Warranties.  Borrower represents and warrants that
          -----------------------------
the Representations and Warranties contained in the Agreement are true and
correct as of the date of this Amendment, and that no Event of Default has
occurred and is continuing.
<PAGE>
 
     4.    MISCELLANEOUS.
           --------------

           (a)  Successors and Assigns.  This Amendment shall be binding upon 
                ----------------------
and shall inure to the benefit of Borrower and Bank and their respective
successors and assigns; provided, however, that the foregoing shall not
authorize any assignment by Borrower of its rights or duties hereunder.

          (b)  Entire Agreement.  This Amendment and the Loan Documents 
               ---------------- 
contain the entire agreement of the parties hereto and supersede any other
oral or written agreements or understandings.

          (c)  Course of Dealing; Waivers.  No course of dealing on the part 
               --------------------------
of Bank or its officers, nor any failure or delay in the exercise of any right
by Bank, shall operate as a waiver thereof, and any single or partial exercise
of any such right shall not preclude any later exercise of any such right.
Bank's failure at any time to require strict performance by Borrower of any
provision shall not affect any right of Bank thereafter to demand strict
compliance and performance. Any suspension or waiver of a right must be in
writing signed by an officer of Bank.

          (d)  Legal Effect.  Except as amended by this Amendment, the Loan 
               ------------
Documents remain in full force and effect. If any provision of this Amendment
conflicts with applicable law, such provision shall be deemed severed from
this Amendment, and the balance of this Amendment shall remain in full force
and effect. Unless otherwise defined, all capitalized terms in this Amendment
shall have the meaning set forth in the Agreement.

          (e)  Counterparts.  This Amendment may be executed in two or more 
               ------------
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one instrument.

     IN WITNESS WHEREOF, the undersigned have executed this Amendment as of the
first date above written.


                                        INKTOMI CORPORATION
 
 
                                        By
                                          ----------------------------------

                                        Title
                                             -------------------------------
 
 
 
                                        SILICON VALLEY BANK
 
 
                                        By
                                          ----------------------------------

                                        Title
                                             -------------------------------
<PAGE>
 
                                  EXHIBIT C

                           COMPLIANCE CERTIFICATE

 
TO:    SILICON VALLEY BANK
 
FROM:  INKTOMI CORPORATION

     The undersigned authorized officer of Inktomi Corporation hereby certifies
that in accordance with the terms and conditions of the Amended and Restated
Loan and Security Agreement between Borrower and Bank (the "Agreement"), (i)
Borrower is in complete compliance for the period ending ___________ with all
required covenants except as noted below and (ii) all representations and
warranties of Borrower stated in the Agreement are true and correct in all
material respects as of the date hereof.  Attached herewith are the required
documents supporting the above certification.  The Officer further certifies
that these are prepared in accordance with Generally Accepted Accounting
Principles (GAAP) and are consistently applied from one period to the next
except as explained in an accompanying letter or footnotes.

 Please indicate compliance status by circling Yes/No under "Complies" column.

<TABLE>
<CAPTION>

Reporting Covenant                   Required                                   Complies
------------------                   --------                                   --------
<S>                             <C>                                          <C>
10-Q & CC                            Quarterly within 45 days                   Yes       No
10-K & CC                            FYE within 120 days                        Yes       No
A/R & A/P Agings                     Within 20 days of request                  Yes       No
</TABLE>

<TABLE>
<CAPTION>
Financial Covenant                   Required                 Actual            Complies
------------------                   --------                 ------            -------
<S>                               <C>                   <C>                   <C>
Maintain on a Quarterly Basis:
 Minimum Quick Ratio                  2.0:1.0(1)               _____:1.0        Yes       No
 Liquidity Ratio(2)                   2.0:1.0                  _____:1.0        Yes       No
 Debt Service Coverage(3)             1.5:1.0                  _____:1.0        Yes       No
 Minimum Tangible Net Worth          $_________(4)            $________
</TABLE>

(1) Quick Assets excludes Microsoft Accounts that service Microsoft Debt;
Current Liabilities excludes deferred revenues and Microsoft Debt.
(2) Converts to Debt Service Coverage upon two consecutive quarters of DSC of at
least 1.50:1.0.
(3) Tested after conversion of Liquidity Ratio.
(4) $30,000,000 plus (i) 75% of net income (but not loss) for each fiscal
quarter commencing with quarter ended March 30, 1999, plus (ii) 100% of the Net
Proceeds of any Equity Issuance after the date hereof.

Comments Regarding Exceptions:  See Attached.
Sincerely,

 
-----------------------------
SIGNATURE

 
-----------------------------
TITLE

 
-----------------------------
DATE
<PAGE>
 
                        CORPORATE RESOLUTIONS TO BORROW


Borrower:  Inktomi Corporation



     I, the undersigned Secretary or Assistant Secretary of Inktomi Corporation
(the "Corporation"), HEREBY CERTIFY that the Corporation is organized and
existing under and by virtue of the laws of the state of California.

     I FURTHER CERTIFY that the Articles of Incorporation and By Laws previously
delivered to Silicon Valley Bank (the "Bank") remain in full force and effect
and have not been amended, restated or modified.

     I FURTHER CERTIFY that at a meeting of the Directors of the Corporation
duly called and held, at which a quorum was present and voting (or by other duly
authorized corporate action in lieu of a meeting), the following resolutions
were adopted.

     BE IT RESOLVED, that any one (1) of the following named officers,
employees, or agents of this Corporation, whose actual signatures are shown
below:

  NAMES                    POSITIONS            ACTUAL SIGNATURES
  -----                    ---------            -----------------











acting for an on behalf of this Corporation and as its act and deed be, and they
hereby are, authorized and empowered:

     Borrow Money. To borrow from time to time from Bank, on such terms as may
be agreed upon between the officers, employees, or agents and Bank, such sum or
sums of money as in their judgment should be borrowed, without limitation,
including such sums as are specified in that certain Amendment to Amended and
Restated Loan and Security Agreement dated as of January 28, 1999 (the
"Amendment").

     Execute Notes.  To execute and deliver to Bank the Amendment and also to
execute and deliver to Bank one or more renewals, extensions, modifications,
refinancings, consolidations, or substitutions for one or more of the notes, or
any portion of the notes.

     Grant Security.  To grant a security interest to Bank in the Collateral
described in the Loan Agreement, which security interest shall secure all of the
Corporation's Obligations, as described in the Loan Agreement.

     Negotiate Items. To draw, endorse, and discount with Bank all drafts, trade
acceptances, promissory notes, or other evidences of indebtedness payable to or
belonging to the Corporation or in which the Corporation may have an interest,
and either to receive cash for the same or to cause such proceeds to be credited
to the account of the Corporation with Bank, or to cause such other disposition
of the proceeds derived therefrom as they may deem advisable.

     Letters of Credit; Foreign Exchange.  To execute letters of credit
applications, foreign exchange agreements and other related documents pertaining
to Bank's issuance of letters of credit and foreign exchange contracts.
<PAGE>
 
     Further Acts.  In the case of lines of credit, to designate additional or
alternate individuals as being authorized to request advances thereunder, and in
all cases, to do and perform such other acts and things, to pay any and all fees
and costs, and to execute and deliver such other documents and agreements as
they may in their discretion deem reasonably necessary or proper in order to
carry into effect the provisions of these Resolutions.

     BE IT FURTHER RESOLVED, that any and all acts authorized pursuant to these
resolutions and performed prior to the passage of these resolutions are hereby
ratified and approved, that these Resolutions shall remain in full force and
effect and Bank may rely on these Resolutions until written notice of their
revocation shall have been delivered to and received by Bank.  Any such notice
shall not affect any of the Corporation's agreements or commitments in effect at
the time notice is given.

     I FURTHER CERTIFY that the officers, employees, and agents named above are
duly elected, appointed, or employed by or for the Corporation, as the case may
be, and occupy the positions set forth opposite their respective names; that the
foregoing Resolutions now stand of record on the books of the Corporation; and
that the Resolutions are in full force and effect and have not been modified or
revoked in any manner whatsoever.

     IN WITNESS WHEREOF, I have hereunto set my hand on _________________, 1999
and attest that the signatures set opposite the names listed above are their
genuine signatures.


                                    CERTIFIED TO AND ATTESTED BY:


                                    X