Document:

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                                                                   EXHIBIT 10.25

                       FIRST LOAN MODIFICATION AGREEMENT

     This First Loan Modification Agreement is entered into as of March 8, 2000,
by and between NATURAL MICROSYSTEMS CORPORATION, a Delaware corporation with its
chief executive office located at 100 Crossing Boulevard, Framingham,
Massachusetts ("Borrower") and SILICON VALLEY BANK, a California-chartered bank
("Bank"), with its principal place of business at 3003 Tasman Drive, Santa
Clara, CA 95054 and with a loan production office located at Wellesley Office
Park, 40 William Street, Suite 350, Wellesley, MA 02481, doing business under
the name "Silicon Valley East".

1.   DESCRIPTION OF EXISTING INDEBTEDNESS. Among other indebtedness which may be
owing by Borrower to Bank, Borrower is indebted to Bank pursuant to a loan
arrangement dated as of May 14, 1999, evidenced by, among other documents, a
certain Loan and Security Agreement dated as of May 14, 1999 (the "Loan
Agreement").  The Loan Agreement established a working capital line of credit in
favor of the Borrower in the maximum principal amount of Seven Million Five
Hundred Thousand Dollars ($7,500,000.00) (the "Committed Revolving Line").
Capitalized terms used but not otherwise defined herein shall have the same
meaning as in the Loan Agreement.

Hereinafter, all indebtedness owing by Borrower to Bank shall be referred to as
the "Indebtedness".

2.   DESCRIPTION OF COLLATERAL.  Repayment of the Indebtedness is secured by the
Collateral as described in the Loan Agreement (together with any other
collateral security granted to Bank, the "Security Documents").

Hereinafter, the Security Documents, together with all other documents
evidencing or securing the Indebtedness shall be referred to as the "Existing
Loan Documents".

3.   DESCRIPTION OF CHANGE IN TERMS.

     A.   Modification(s) to Loan Agreement.

          1.   The Loan Agreement shall be amended by deleting the following
               definition appearing in Section 1.1 thereof:

                    ""Credit Extension" means each Advance, or any other
                    extension of credit by Bank for the benefit of Borrower
                    hereunder."

               and inserting in lieu thereof the following:

                    ""Credit Extension" means each Advance, Letter of Credit,
                    Exchange Contract, Cash Management Services or any other
                    extension of credit by Bank for the benefit of Borrower
                    hereunder."

          2.   The Loan Agreement shall be amended by deleting the following
               definition appearing in Section 1.1 thereof:

                    ""Tangible Net Worth" means as of any applicable date, the
                    consolidated total assets of Borrower and its Subsidiaries
                    minus, without duplication, (i) the sum of any amounts
                    attributable to (a) goodwill, (b) intangible items such as
                    unamortized debt discount and expense, patents, trade and
                    service marks and names, copyrights and research and
                    development expenses except prepaid expenses, and (c) all
                    reserves not already deducted from assets, and (ii) Total
                    Liabilities; and (iii) Other Assets."

                                      -1-
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               and inserting in lieu thereof the following:

                    ""Tangible Net Worth" means as of any applicable date, the
                    consolidated total assets of Borrower and its Subsidiaries
                    minus, without duplication, (i) the sum of any amounts
                    attributable to (a) goodwill, (b) intangible items such as
                    unamortized debt discount and expense, patents, trade and
                    service marks and names, copyrights and research and
                    development expenses except prepaid expenses, and (c) all
                    reserves not already deducted from assets, and (ii) Total
                    Liabilities; and (iii) Other Assets, and (iv) tax assets,
                    and (v) security deposits."

          3.   The Loan Agreement shall be amended by incorporating the
               following definitions into Section 1.1 thereof:

                    "Cash Management Services" are defined in Section 2.1.4.

                    "Exchange Contract" has the meaning set forth in Section
                    2.1.3.

                    "Letter of Credit" means a letter of credit or similar
                    undertaking issued by Bank pursuant to Section 2.1.2.

                    "Letter of Credit Reserve" has the meaning set forth in
                    Section 2.1.2.

                    "Foreign Exchange Reserve" has the meaning set forth in
                    Section 2.1.3.

          4.   The Loan Agreement shall be amended by deleting the following
               text appearing as the first sentence of paragraph (a) of Section
               2.1.1 entitled "Credit Extensions":

                    "Subject to and upon the terms and conditions of this
                    Agreement, Bank agrees to make Advances to Borrower in an
                    aggregate outstanding amount not to exceed the Committed
                    Revolving Line or the Borrowing Base, whichever is less."

               and inserting in lieu thereof the following:

                    "Subject to and upon the terms and conditions of this
                    Agreement, Bank agrees to make Advances to Borrower in an
                    aggregate outstanding amount not to exceed: (i) the
                    Committed Revolving Line or the Borrowing Base, whichever is
                    less, minus (ii) the face amount of all outstanding Letters
                    of Credit (including drawn but unreimbursed Letters of
                    Credit), minus (iii) the Foreign Exchange Reserve, minus
                    (iv) the outstanding Cash Management Services, and minus (v)
                    the aggregate outstanding Advances hereunder."

          5.   The Loan Agreement shall be amended by inserting after Section
               2.1.1 thereof the following new section:

                    "2.1.2  Letters of Credit.

                                      -2-
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                    (a)   Subject to the terms and conditions of this Agreement,
                    Bank agrees to issue or cause to be issued Letters of Credit
                    for the account of Borrower in an aggregate outstanding face
                    amount not to exceed (i) the Committed Revolving Line or the
                    Borrowing Base, whichever is less, minus (ii) the Foreign
                    Exchange Reserve, minus (iii) the outstanding Cash
                    Management Services, and minus (iv) the then outstanding
                    principal balance of the Advances; provided that the face
                    amount of outstanding Letters of Credit (including drawn but
                    unreimbursed Letters of Credit and any Letter of Credit
                    Reserve) shall not in any case exceed Seven Million Five
                    Hundred Thousand Dollars ($7,500,000.00).  Each Letter of
                    Credit shall have an expiry date no later than one hundred
                    eighty (180) days after the Maturity Date provided that
                    Borrower's Letter of Credit reimbursement obligation shall
                    be secured by cash on terms acceptable to Bank at any time
                    after the Maturity Date if the term of this Agreement is not
                    extended by Bank.  All  Letters of Credit shall be, in form
                    and substance, acceptable to Bank in its sole discretion and
                    shall be subject to the terms and conditions of Bank's form
                    of standard Application and Letter of Credit Agreement.

                    (b)   The obligation of Borrower to immediately reimburse
                    Bank for drawings made under Letters of Credit shall be
                    absolute, unconditional and irrevocable, and shall be
                    performed strictly in accordance with the terms of this
                    Agreement and such Letters of Credit, under all
                    circumstances whatsoever.  Borrower shall indemnify, defend,
                    protect, and hold Bank harmless from any loss, cost, expense
                    or liability, including, without limitation, reasonable
                    attorneys' fees, arising out of or in connection with any
                    Letters of Credit.

                    (c)   Borrower may request that Bank issue a Letter of
                    Credit payable in a currency other than United States
                    Dollars.  If a demand for payment is made under any such
                    Letter of Credit, Bank shall treat such demand as an Advance
                    to Borrower of the equivalent of the amount thereof (plus
                    cable charges) in United States currency at the then
                    prevailing rate of exchange in San Francisco, California,
                    for sales of that other currency for cable transfer to the
                    country of which it is the currency.

                    (d)   Upon the issuance of any letter of credit payable in a
                    currency other than United States Dollars, Bank shall create
                    a reserve (the "Letter of Credit Reserve") under the
                    Committed Revolving Line for letters of credit against
                    fluctuations in currency exchange rates, in an amount equal
                    to ten percent (10%) of the face amount of such letter of
                    credit.  The amount of such reserve may be amended by Bank
                    from time to time to account for fluctuations in the
                    exchange rate.  The availability of funds under the
                    Committed Revolving Line shall be reduced by the amount of
                    such reserve for so long as such letter of credit remains
                    outstanding."

                                      -3-
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          6.   The Loan Agreement shall be amended by inserting after Section
               2.1.2 thereof the following new section:

                    "2.1.3  Foreign Exchange Contract; Foreign Exchange
                            Settlements.

                    (a)   Subject to the terms of this Agreement, Borrower may
                    enter into foreign exchange contracts (the "Exchange
                    Contracts") not to exceed an aggregate amount of Seven
                    Million Five Hundred Thousand Dollars ($7,500,000.00) (the
                    "Contract Limit"), pursuant to which Bank shall sell to or
                    purchase from Borrower foreign currency on a spot or future
                    basis.  Borrower shall not request any Exchange Contracts at
                    any time it is out of compliance with any of the provisions
                    of this Agreement.  All Exchange Contracts must provide for
                    delivery of settlement on or before the Maturity Date.  The
                    amount available under the Committed Revolving Line at any
                    time shall be reduced by the following amounts (the "Foreign
                    Exchange Reserve") on any given day (the "Determination
                    Date"):  (i) on all outstanding Exchange Contracts on which
                    delivery is to be effected or settlement allowed more than
                    two business days after the Determination Date, 10% of the
                    gross amount of the Exchange Contracts; plus (ii) on all
                    outstanding Exchange Contracts on which delivery is to be
                    effected or settlement allowed within two business days
                    after the Determination Date, 100% of the gross amount of
                    the Exchange Contracts.

                    (b)   Bank may, in its discretion, terminate the Exchange
                    Contracts at any time (i) that an Event of Default occurs or
                    (ii) that there is no sufficient availability under the
                    Committed Revolving Line and Borrower does not have
                    available funds in its bank account to satisfy the Foreign
                    Exchange Reserve.  If Bank terminates the Exchange
                    Contracts, and without limitation of any applicable
                    indemnities, Borrower agrees to reimburse Bank for any and
                    all fees, costs and expenses relating thereto or arising in
                    connection therewith.

                    (c)   Borrower shall not permit the total gross amount of
                    all Exchange Contracts on which delivery is to be effected
                    and settlement allowed in any two business day period to be
                    more than One Million Dollars ($1,000,000.00) (the
                    "Settlement Limit") nor shall Borrower permit the total
                    gross amount of all Exchange Contracts to which Borrower is
                    a party, outstanding at any one time, to exceed the Contract
                    Limit.  Notwithstanding the above, however, the amount which
                    may be settled in any two (2) business day period may be
                    increased above the Settlement Limit up to, but in no event
                    to exceed, the amount of the Contract Limit under either of
                    the following circumstances:

                         (i)   if there is sufficient availability under the
                         Committed Revolving Line in the amount of the Foreign
                         Exchange Reserve as of each Determination Date,
                         provided that Bank in advance shall reserve the full
                         amount of the Foreign Exchange Reserve against the
                         Committed Revolving Line; or

                         (ii)   if there is insufficient availability under the
                         Committed Revolving Line, as to settlements within any
                         two (2) business day period, provided that Bank, in its
                         sole discretion, may:  (A) verify good funds overseas
                         prior to crediting Borrower's deposit account with Bank
                         (in the case of Borrower's sale of foreign currency);
                         or (B) debit Borrower's deposit account with Bank prior
                         to delivering foreign currency overseas (in the case of
                         Borrower's purchase of foreign currency).

                    (d)   In the case of Borrower's purchase of foreign
                    currency, Borrower in advance shall instruct Bank upon
                    settlement either to treat the settlement amount as an
                    advance under the Committed Revolving Line, or to debit
                    Borrower's account for the amount settled.

                                      -4-
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                    (e)   Borrower shall execute all standard from applications
                    and agreements of Bank in connection with the Exchange
                    Contracts and, without limiting any of the terms of such
                    applications and agreements, Borrower will pay all standard
                    fees and charges of Bank in connection with the Exchange
                    Contracts.

                    (f)   Without limiting any of the other terms of this
                    Agreement or any such standard form applications and
                    agreement of Bank, Borrower agrees to indemnify Bank and
                    hold it harmless, from and against any and all claims,
                    debts, liabilities, demands, obligations, actions, costs and
                    expenses (including, without limitation, attorneys' fees of
                    counsel of Bank's choice), of every nature and description
                    which it may sustain or incur, based upon, arising out of,
                    or in any way relating to any of the Exchange Contracts or
                    any transactions relating thereto or contemplated thereby."

          7.   The Loan Agreement shall be amended by inserting after Section
               2.1.3 thereof the following new section:

                    "2.1.4   Cash Management Sublimit.  Borrower may use up to
                    Seven Million Five Hundred Thousand Dollars ($7,500,000.00)
                    for Bank's Cash Management Services, which may include
                    merchant services, direct deposit of payroll, business
                    credit card, and check cashing services identified in the
                    Cash Management Services Agreement (the "Cash Management
                    Services").  All amounts Bank pays for any Cash Management
                    Services shall be treated as an Advance under the Committed
                    Revolving Line."

          8.   The Loan Agreement shall be amended by deleting the following
               text appearing as Section 2.2 entitled "Overadvances":

                    "2.2   Overadvances.  If, at any time or for any reason, the
                    amount of Obligations owed by Borrower to Bank pursuant to
                    Section 2.1.1 of this Agreement is greater than the lesser
                    of (i) the Committed Revolving Line or (ii) the Borrowing
                    Base, Borrower shall immediately pay to Bank, in cash, the
                    amount of such excess."

               and inserting in lieu thereof the following:

                    "2.2   Overadvances.  If, at any time or for any reason, the
                    amount of Obligations owed by Borrower to Bank pursuant to
                    Section 2.1.1, 2.1.2, 2.1.3 and 2.1.4 of this Agreement is
                    greater than: (i) the Committed Revolving Line or the
                    Borrowing Base, whichever is less, minus (ii) the face
                    amount of all outstanding Letters of Credit (including drawn
                    but unreimbursed Letters of Credit), minus (iii) the Foreign
                    Exchange Reserve, and minus (iv) the outstanding Cash
                    Management Services, Borrower shall immediately pay to Bank,
                    in cash, the amount of such excess."

          9.   The Loan Agreement shall be amended by deleting the following
               text appearing as paragraph (a) of Section 2.3 entitled "Interest
               Rate":

                    "(a)   Interest Rate.  Except as set forth in Section
                    2.3(b), any Advances shall bear interest, on the average
                    daily balance thereof, at a per annum rate equal to the
                    aggregate of the Bank's Prime Rate plus one percent (1.0%)
                    per annum."

                                      -5-
<PAGE>

               and inserting in lieu thereof the following:

                    "(a)   Interest Rate.  Except as set forth in Section
                    2.3(b), any Advances shall bear interest, on the average
                    daily balance thereof, at a per annum rate equal to the
                    aggregate of the Bank's Prime Rate, plus one percent (1.0%).
                    Notwithstanding the foregoing, upon the completion of a
                    secondary public offering of Borrower's stock, any Advances
                    shall bear interest (except as set forth in Section 2.3(b)),
                    on the average daily balance thereof, at a per annum rate
                    equal to the Bank's Prime Rate."

          10.  For purposes of the definition of Unused Committed Revolving Line
               Facility Fee appearing in Section 2.5(a), all issued but undrawn
               Letters of Credit shall be considered "advanced".

          11.  The Loan Agreement shall be amended by deleting Sections 6.8, 6.9
               and 6.10 in their entirety and inserting in lieu thereof the
               following:

                    "6.8   Quick Ratio.  Borrower shall maintain, as of the last
                    day of each calendar month, commencing with the month ending
                    December 31, 1999, a ratio of Quick Assets to Current
                    Liabilities of at least 1.25 to 1.0.  Notwithstanding the
                    foregoing, upon the completion of a secondary public
                    offering of the Borrower's stock, Borrower shall maintain,
                    as of the last day of each calendar month, a ratio of Quick
                    Assets to Current Liabilities of at least 2.0 to 1.0.

                    6.9   Tangible Net Worth.  Borrower shall maintain, as of
                    the last day of each calendar quarter, commencing with the
                    quarter ending December 31, 1999, a Tangible Net Worth of
                    not less than the aggregate of: (i) Twenty-Eight Million
                    Dollars ($28,000,000.00), plus (ii) fifty percent (50%) of
                    the amount of cash received by Borrower from a public
                    offering of the Borrower's stock, plus (iii) fifty percent
                    (50%) of Borrower's quarterly net income.

                    6.10   Profitability.  Borrower shall maintain, on a
                    quarterly basis: (i) a net loss of no greater than Thirteen
                    Million Dollars ($13,000,000.00) as of the last day of the
                    Fourth Quarter of 1999, and (ii) a net loss of no greater
                    than Three Million Five Hundred Thousand Dollars
                    ($3,500,000.00) as of the last day of the First Quarter of
                    2000."

          12.  The Bank hereby consents to Borrower's acquisition of QUES.com.
               Except as specifically set forth in the foregoing consent, the
               Bank hereby reserves all of its rights and remedies arising under
               the Loan Agreement, as amended hereby.

          13.  The Loan Agreement shall be amended in Section 9.1 entitled
               "Rights and Remedies" by incorporating therein immediately after
               paragraph (i) the following two paragraphs:

                    "(j)   Demand that Borrower (i) deposit cash with Bank in an
                    amount equal to the amount of any Letters of Credit
                    remaining undrawn, as collateral security for the repayment
                    of any future drawings under such Letters of Credit, and
                    Borrower shall forthwith deposit and pay such amounts, and
                    (ii) pay in advance all Letters of Credit fees scheduled to
                    be paid or payable over the remaining term of the Letters of
                    Credit; and

                                      -6-
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                    (k)   Liquidate any Exchange Contracts not yet settled and
                    demand that Borrower immediately deposit cash with Bank in
                    an amount sufficient to cover any losses incurred by Bank
                    due to liquidation of the Exchange Contracts at the then
                    prevailing market price."

          14.  The Borrower hereby ratifies, confirms and reaffirms, all and
               singular, the terms and conditions of a certain Negative Pledge
               Agreement dated as of May 14, 1999, between Borrower and Bank,
               and acknowledges, confirms and agrees that said Negative Pledge
               Agreement shall remain in full force and effect.

          15.  The Borrowing Base Certificate appearing as EXHIBIT C to the Loan
               Agreement is hereby replaced with the Borrowing Base Certificate
               attached as EXHIBIT A hereto.

          16.  The Compliance Certificate appearing as EXHIBIT D to the Loan
               Agreement is hereby replaced with the Compliance Certificate
               attached as EXHIBIT B hereto.

4.   FEE.  The Borrower shall reimburse Bank for all legal fees and expenses
incurred in connection with this amendment to the Existing Loan Documents.

5.   CONSISTENT CHANGES.  The Existing Loan Documents are hereby amended
wherever necessary to reflect the changes described above.

6.   RATIFICATION OF LOAN DOCUMENTS.  Borrower hereby ratifies, confirms, and
reaffirms all terms and conditions of all security or other collateral granted
to the Bank, and confirms that the indebtedness secured thereby includes,
without limitation, the Indebtedness.

7.   NO DEFENSES OF BORROWER.  Borrower agrees that, as of this date, it has no
defenses against the obligations to pay any amounts under the Indebtedness.

8.   CONTINUING VALIDITY.  Borrower understands and agrees that in modifying the
existing Indebtedness, Bank is relying upon Borrower's representations,
warranties, and agreements, as set forth in the Existing Loan Documents.  Except
as expressly modified pursuant to this Loan Modification Agreement, the terms of
the Existing Loan Documents remain unchanged and in full force and effect.
Bank's agreement to modifications to the existing Indebtedness pursuant to this
Loan Modification Agreement in no way shall obligate Bank to make any future
modifications to the Indebtedness.  Nothing in this Loan Modification Agreement
shall constitute a satisfaction of the Indebtedness.  It is the intention of
Bank and Borrower to retain as liable parties all makers of Existing Loan
Documents, unless the party is expressly released by Bank in writing.  No maker
will be released by virtue of this Loan Modification Agreement.

9.   JURISDICTION/VENUE.  Borrower accepts for itself and in connection with its
properties, unconditionally, the non-exclusive jurisdiction of any state or
federal court of competent jurisdiction in the Commonwealth of Massachusetts in
any action, suit, or proceeding of any kind against it which arises out of or by
reason of this Loan Modification Agreement; provided, however, that if for any
reason Bank cannot avail itself of the courts of the Commonwealth of
Massachusetts, then venue shall lie in Santa Clara County, California.

10.  COUNTERSIGNATURE.  This Loan Modification Agreement shall become effective
only when it shall have been executed by Borrower and Bank (provided, however,
in no event shall this Loan Modification Agreement become effective until signed
by an officer of Bank in California).

                                      -7-
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     This Loan Modification Agreement is executed as a sealed instrument under
the laws of the Commonwealth of Massachusetts as of the date first written
above.

BORROWER:                              BANK:

NATURAL MICROSYSTEMS CORPORATION       SILICON VALLEY BANK, doing business as
                                       SILICON VALLEY EAST

By:                                    By:
   -----------------------------          ----------------------------------
Name:                                  Name:
     ---------------------------            --------------------------------
Title:                                 Title:
      --------------------------             -------------------------------

                                       SILICON VALLEY BANK

                                       By:
                                          ----------------------------------

                                       Name:
                                            --------------------------------

                                       Title:
                                             -------------------------------
                                             (signed in Santa Clara County,
                                              California)

                                      -8-<PAGE>

                                                                   EXHIBIT 10.26

                      SECOND LOAN MODIFICATION AGREEMENT

     This Second Loan Modification Agreement is entered into as of September 14,
2000, by and between NATURAL MICROSYSTEMS CORPORATION, a Delaware corporation
with its chief executive office located at 100 Crossing Boulevard, Framingham,
Massachusetts ("Borrower") and SILICON VALLEY BANK, a California-chartered bank
("Bank"), with its principal place of business at 3003 Tasman Drive, Santa
Clara, CA 95054 and with a loan production office located at Wellesley Office
Park, 40 William Street, Suite 350, Wellesley, MA 02481, doing business under
the name "Silicon Valley East".

1.   DESCRIPTION OF EXISTING INDEBTEDNESS.  Among other Obligations which may be
owing by Borrower to Bank, Borrower is indebted to Bank pursuant to a loan
arrangement dated as of May 14, 1999, evidenced by, among other documents, a
certain Loan and Security Agreement dated as of May 14, 1999, as amended by a
certain First Loan Modification Agreement dated March 8, 2000 (as amended, the
"Loan Agreement").  The Loan Agreement established a working capital line of
credit in favor of the Borrower in the maximum principal amount of Seven Million
Five Hundred Thousand Dollars ($7,500,000.00) (the "Committed Revolving Line").
Capitalized terms used but not otherwise defined herein shall have the same
meaning as in the Loan Agreement.

Hereinafter, all indebtedness owing by Borrower to Bank shall be referred to as
the "Obligations".

2.   DESCRIPTION OF COLLATERAL.  Repayment of the Obligations is secured by the
Collateral as described in the Loan Agreement (together with any other
collateral security granted to Bank, the "Security Documents").

Hereinafter, the Security Documents, together with all other documents
evidencing or securing the Obligations shall be referred to as the "Existing
Loan Documents".

3.   DESCRIPTION OF CHANGE IN TERMS.

     A.   Modification(s) to Loan Agreement.

          1.   The Loan Agreement shall be amended by deleting the following
               definition appearing in Section 1.1 thereof:

                    ""Maturity Date" means one day prior to one year from the
                    Closing Date."

               and inserting in lieu thereof the following:

                    ""Maturity Date" means May 13, 2001."

          2.   The Loan Agreement shall be amended by deleting the following
               text appearing as paragraph (a) of Section 2.3 entitled "Interest
               Rate":

                    "(a)   Interest Rate.  Except as set forth in Section
                    2.3(b), any Advances shall bear interest, on the average
                    daily balance thereof, at a per annum rate equal to the
                    aggregate of the Bank's Prime Rate, plus one percent (1.0%).
                    Notwithstanding the foregoing, upon the completion of a
                    secondary public offering of Borrower's stock, any Advances
                    shall bear interest (except as set forth in Section 2.3(b)),
                    on the average daily balance thereof, at a per annum rate
                    equal to the Bank's Prime Rate."

               and inserting in lieu thereof the following:

                                      53
<PAGE>

                    "(a)   Interest Rate.  Except as set forth in Section
                    2.3(b), any Advances shall bear interest, on the average
                    daily balance thereof, at a per annum rate equal to the
                    Bank's Prime Rate."

          3.   The Loan Agreement shall be amended by deleting Section 6.3
               entitled "Financial Statements, Reports, Certificates" and
               inserting in lieu thereof the following:

                    "6.3  Financial Statements, Reports, Certificates.  Borrower
                    shall deliver to Bank:  (a) within five (5) days of filing
                    same quarterly with the Securities and Exchange Commission
                    (the "SEC"), a company prepared consolidated balance sheet
                    and income statement covering Borrower's consolidated
                    operations during such period, in a form and certified by an
                    officer of Borrower reasonably acceptable to Bank; (b)
                    within five (5) days of filing same annually with the SEC,
                    audited consolidated financial statements of Borrower
                    prepared in accordance with GAAP, consistently applied,
                    together with an unqualified opinion on such financial
                    statements of an independent certified public accounting
                    firm reasonably acceptable to Bank; (c) within five (5) days
                    of filing, copies of all statements, reports and notices
                    sent or made available generally by Borrower to its security
                    holders or to any holders of Subordinated Debt and all
                    reports on Form 10-K, 10-Q and 8-K filed with the Securities
                    and Exchange Commission; (d) promptly upon receipt of notice
                    thereof, a report of any legal actions pending or threatened
                    against Borrower or any Subsidiary that could result in
                    damages or costs to Borrower or any Subsidiary of One
                    Hundred Thousand Dollars ($100,000) or more; (e) within
                    thirty (30) days of the approval thereof, any budgets or
                    forecasts or revisions thereto; and (f) such budgets, sales
                    projections, operating plans or other financial information
                    as Bank may reasonably request from time to time.

                         Within fifteen (15) days after the last day of each
                    quarter during which Advances are requested or outstanding
                    (excluding Letters of Credit), and in connection with any
                    Advance (excluding Letters of Credit) (unless a Borrowing
                    Base Certificate has been provided within the last thirty
                    (30) days), Borrower shall deliver to Bank a Borrowing Base
                    Certificate signed by a Responsible Officer in substantially
                    the form of Exhibit C hereto, together with an aged listing
                    of accounts receivable (by invoice date).

                         Borrower shall deliver to Bank with the quarterly
                    financial statements, within five (5) days of filing same
                    quarterly with the SEC, a Compliance Certificate signed by a
                    Responsible Officer in substantially the form of Exhibit D
                    hereto.

                         Bank shall have a right from time to time hereafter to
                    audit Borrower's Accounts at Borrower's expense, provided
                    that such audits will be conducted no more often than every
                    twelve (12) months unless an Event of Default has occurred
                    and is continuing."

           4.   The Loan Agreement shall be amended by deleting the following
                text appearing as Sections 6.8, 6.9 and 6.10 thereof:
<PAGE>

                    "6.8   Quick Ratio.  Borrower shall maintain, as of the last
                    day of each calendar month, commencing with the month ending
                    December 31, 1999, a ratio of Quick Assets to Current
                    Liabilities of at least 1.25 to 1.0.  Notwithstanding the
                    foregoing, upon the completion of a secondary public
                    offering of the Borrower's stock, Borrower shall maintain,
                    as of the last day of each calendar month, a ratio of Quick
                    Assets to Current Liabilities of at least 2.0 to 1.0.

                    6.9   Tangible Net Worth.  Borrower shall maintain, as of
                    the last day of each calendar quarter, commencing with the
                    quarter ending December 31, 1999, a Tangible Net Worth of
                    not less than the aggregate of: (i) Twenty-Eight Million
                    Dollars ($28,000,000.00), plus (ii) fifty percent (50%) of
                    the amount of cash received by Borrower from a public
                    offering of the Borrower's stock, plus (iii) fifty percent
                    (50%) of Borrower's quarterly net income.

                    6.10   Profitability.  Borrower shall maintain, on a
                    quarterly basis: (i) a net loss of no greater than Thirteen
                    Million Dollars ($13,000,000.00) as of the last day of the
                    Fourth Quarter of 1999, and (ii) a net loss of no greater
                    than Three Million Five Hundred Thousand Dollars
                    ($3,500,000.00) as of the last day of the First Quarter of
                    2000."

               and inserting in lieu thereof the following:

                    "6.8   Quick Ratio.  Borrower shall maintain, as of the last
                    day of each  quarter, a ratio of Quick Assets to Current
                    Liabilities of at least 2.0 to 1.0.

                    6.9   [Intentionally Deleted]

                    6.10   Profitability.  Borrower shall maintain, on a
                    quarterly basis, commencing with the quarter ending
                    September 30, 2000, an operating profit of not less than One
                    Dollar ($1.00), which amount shall be exclusive of certain
                    acquisition costs approved by Bank, which shall include
                    merger and acquisition costs and goodwill attributed thereto
                    as referenced on Borrower's Forecast Plan attached hereto as
                    EXHIBIT E."

          5.   The Bank hereby consents to Borrower's recent acquisition of
               InnoMediaLogic, Inc. Except as specifically set forth in the
               foregoing consent, the Bank hereby reserves all of its rights and
               remedies arising under the Loan Agreement, as amended hereby.

          6.   The Loan Agreement shall be amended by deleting the following
               text appearing as Section 7.3 thereof:

                    "7.3  Mergers or Acquisitions.  Merge or consolidate, or
                    permit any of its Subsidiaries to merge or consolidate, with
                    or into any other business organization, or acquire, or
                    permit any of its Subsidiaries to acquire, all or
                    substantially all of the capital stock or property of
                    another Person."

               and inserting in lieu thereof the following:

                    "7.3  Mergers or Acquisitions.  Merge or consolidate, or
                    permit any of its Subsidiaries to merge or consolidate, with
                    or into any other business
<PAGE>

                    organization, or acquire, or permit any of its Subsidiaries
                    to acquire, all or substantially all of the capital stock or
                    property of another Person EXCEPT that such merger,
                    consolidation or acquisition may occur without the written
                    consent of the Bank where: (i) there is no Event of Default
                    then existing hereunder, and (ii) such merger, consolidation
                    or acquisition will not result, on a prospective basis, in
                    the breach of any of the covenants, terms and conditions
                    hereunder, and (iii) such merger, consolidation or
                    acquisition is in the same or similar line of business as
                    the Borrower, and (iv) the purchase price for all such
                    mergers, consolidation and acquisitions (inclusive of any
                    indebtedness of the acquired company assumed by the Borrower
                    in connection with the transaction) is not greater than an
                    aggregate annual amount of Fifty Million Dollars
                    ($50,000,000.00) in cash and/or stock of the Borrower (and
                    no greater than an aggregate amount of Twenty-Five Million
                    Dollars ($25,000,000.00) in cash on an annual basis), and
                    (v) the Borrower is the surviving legal entity, and (vi) no
                    indebtedness (either direct or contingent) is assumed by the
                    Borrower in connection with such merger, consolidation or
                    acquisition with the exception of indebtedness (A) in such
                    amounts which when added to the purchase price shall not
                    exceed the limits set forth in clause (iv) of this Section
                    7.3 and (B) which consists only of trade payables of the
                    acquired company."

          7.   The Borrower hereby ratifies, confirms and reaffirms, all and
               singular, the terms and conditions of a certain Negative Pledge
               Agreement dated as of May 14, 1999, between Borrower and Bank,
               and acknowledges, confirms and agrees that said Negative Pledge
               Agreement shall remain in full force and effect.

          8.   The Borrowing Base Certificate appearing as EXHIBIT C to the Loan
               Agreement is hereby replaced with the Borrowing Base Certificate
               attached as EXHIBIT A hereto.

          9.   The Compliance Certificate appearing as EXHIBIT D to the Loan
               Agreement is hereby replaced with the Compliance Certificate
               attached as EXHIBIT B hereto.

          10.  The Borrower's Forecast Plan as it relates to Borrower's
               Profitability covenant is hereby incorporated into the Loan
               Agreement as EXHIBIT E hereto.

4.   FEE.  The Borrower shall pay to Bank a modification fee equal to Eighteen
Thousand Seven Hundred Fifty Dollars ($18,750.00), which fee shall be due on the
date hereof and shall be deemed fully earned as of the date hereof.  The
Borrower shall also reimburse Bank for all legal fees and expenses incurred in
connection with this amendment to the Existing Loan Documents.

5.   CONSISTENT CHANGES.  The Existing Loan Documents are hereby amended
wherever necessary to reflect the changes described above.

6.   RATIFICATION OF LOAN DOCUMENTS.  Borrower hereby ratifies, confirms, and
reaffirms all terms and conditions of all security or other collateral granted
to the Bank, and confirms that the indebtedness secured thereby includes,
without limitation, the Obligations.

7.   NO DEFENSES OF BORROWER.  Borrower agrees that, as of this date, it has no
defenses against the obligations to pay any amounts under the Obligations.
<PAGE>

8.   CONTINUING VALIDITY.  Borrower understands and agrees that in modifying the
existing Obligations, Bank is relying upon Borrower's representations,
warranties, and agreements, as set forth in the Existing Loan Documents.  Except
as expressly modified pursuant to this Loan Modification Agreement, the terms of
the Existing Loan Documents remain unchanged and in full force and effect.
Bank's agreement to modifications to the existing Obligations pursuant to this
Loan Modification Agreement in no way shall obligate Bank to make any future
modifications to the Obligations.  Nothing in this Loan Modification Agreement
shall constitute a satisfaction of the Obligations.  It is the intention of Bank
and Borrower to retain as liable parties all makers of Existing Loan Documents,
unless the party is expressly released by Bank in writing.  No maker will be
released by virtue of this Loan Modification Agreement.

9.   JURISDICTION/VENUE.  Borrower accepts for itself and in connection with its
properties, unconditionally, the non-exclusive jurisdiction of any state or
federal court of competent jurisdiction in the Commonwealth of Massachusetts in
any action, suit, or proceeding of any kind against it which arises out of or by
reason of this Loan Modification Agreement; provided, however, that if for any
reason Bank cannot avail itself of the courts of the Commonwealth of
Massachusetts, then venue shall lie in Santa Clara County, California.

10.  COUNTERSIGNATURE.  This Loan Modification Agreement shall become effective
only when it shall have been executed by Borrower and Bank (provided, however,
in no event shall this Loan Modification Agreement become effective until signed
by an officer of Bank in California).

     This Loan Modification Agreement is executed as a sealed instrument under
the laws of the Commonwealth of Massachusetts as of the date first written
above.

BORROWER:                           BANK:

NATURAL MICROSYSTEMS CORPORATION     SILICON VALLEY BANK, doing business as
                                     SILICON VALLEY EAST

By:_______________________________   By:____________________________________

Name:_____________________________   Name:__________________________________

Title:____________________________   Title:_________________________________

                                     SILICON VALLEY BANK

                                     By:____________________________________

                                     Name:__________________________________

                                     Title:_________________________________
                                     (signed in Santa Clara County, California)

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