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                                                                     EXHIBIT 4.2

                                                                  EXECUTION COPY

                                PLEDGE AGREEMENT

            This PLEDGE AGREEMENT (this "Pledge Agreement") is made and entered
into as of May 24, 2004 by Thoratec Corporation, a California corporation (the
"Pledgor"), having its principal office at 6035 Stoneridge Drive, Pleasanton,
California 94588, in favor of U.S. Bank National Association, in its capacity as
trustee (the "Trustee") for the holders from time to time (the "Holders") of the
Notes (as defined below), issued by the Pledgor under the Indenture referred to
below. Capitalized terms used and not defined in this Pledge Agreement have the
meanings set forth or referred to in the Indenture.

                             PRELIMINARY STATEMENTS:

            (1) The Pledgor and Merrill Lynch & Co., Merrill Lynch, Pierce,
Fenner & Smith Incorporated as the Initial Purchaser are party to a Purchase
Agreement dated May 18, 2004 (the "Purchase Agreement"), pursuant to which the
Pledgor will issue and sell to the Initial Purchaser $215,154,000 aggregate
principal amount at maturity of Senior Subordinated Convertible Notes due 2034
(the "Initial Notes") and pursuant to which the Pledgor has granted to the
Initial Purchaser an option to purchase all or any part of an additional
$32,273,000 aggregate principal amount at maturity of Senior Subordinated
Convertible Notes due 2034 (the "Option Notes" and, together with the Initial
Notes, the "Notes").

            (2) The Pledgor and the Trustee have entered into that certain
indenture dated as of the date hereof (as amended, restated, supplemented or
otherwise modified from time to time, the "Indenture"), pursuant to which the
Pledgor is issuing the Notes on the date hereof.

            (3) The Pledgor is the beneficial owner of, and desires to pledge
the security entitlements (the "Pledged Security Entitlements") with respect to,
(i) the United States Treasury securities identified by CUSIP number in Schedule
I hereto, and credited to the trust account with U.S. Bank National Association
(the "Account Holder") and held by the Trustee, having ABA No. 091000022,
Account No. 785834000, and entitled "Thoratec Corporation Senior Subordinated
Convertible Notes due 2034, Collateral Pledge Account" (the "Pledged Account")
and (ii) all other financial assets credited from time to time to the Pledged
Account (collectively with the assets described in clause (i) above, the
"Pledged Financial Assets");

            (4) To secure the obligation of the Pledgor under the Indenture and
the Notes to pay all of each of the first six scheduled interest payments on the
Notes when due and to secure repayment of the principal amount and interest on
the Notes and Liquidated Damages (as defined in the Indenture), if any, in the
event that the Notes become due and payable prior to such time as the first six
scheduled interest payments thereon shall have been paid in full (collectively,
the "Obligations"), the Pledgor has agreed (i) to pledge to the Trustee for its
benefit and the ratable benefit of the Holders of the Notes, a security interest
in the Collateral (as defined below) securing the payment and performance by the
Pledgor of all of the Obligations and (ii) to execute and deliver this Pledge
Agreement; the Collateral will be sufficient upon receipt of scheduled interest
and principal payments in respect thereto to provide for the payment of the
first six scheduled interest payments on the Notes;

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            (5) It is a condition precedent to the initial purchase of the Notes
by the Initial Purchaser that the Pledgor shall have executed and delivered this
Pledge Agreement; and

            (6) Unless otherwise defined herein or in the Indenture, terms used
in Article 8 or 9 of the Uniform Commercial Code as in effect in the State of
New York ("UCC") and/or in the Federal Book Entry Regulations (as defined below)
are used in this Pledge Agreement as such terms are defined in such Article 8 or
9 and/or the Federal Book Entry Regulations. The term "Federal Book Entry
Regulations" means (a) the federal regulations contained in Subpart B
("Treasury/Reserve Automated Debt Entry System (TRADES)") governing book-entry
securities consisting of U.S. Treasury bonds, notes and bills and Subpart D
("Additional Provisions") of 31 C.F.R. Part 357, 31 C.F.R. Section 357.2,
Section 357.10 through Section 357.14 and Section 357.41 through Section 357.44
and (b) to the extent substantially identical to the federal regulations
referred to in clause (a) above (as in effect from time to time), the federal
regulations governing other book-entry securities.

                                    AGREEMENT

            NOW, THEREFORE, in consideration of the premises herein contained,
and in order to induce the Initial Purchaser to purchase the Notes, the Pledgor
hereby agrees with the Trustee, for the benefit of the Trustee and for the
ratable benefit of the Holders of the Notes, as follows:

            SECTION 1. Pledge and Grant of Security Interest. The Pledgor hereby
pledges and grants to the Trustee, for its benefit and for the ratable benefit
of the Holders of the Notes, a security interest in and continuing lien on all
of the Pledgor's right, title and interest in and to the property set out in
paragraphs (a) through (d) below (in each case, whether now owned or hereafter
acquired or created by the Pledgor, wherever located and whether now or
hereafter existing or arising and whether consisting of security accounts,
securities (including United States Treasury securities), security entitlements,
financial assets and other investment property, deposit accounts, payment
intangibles and other general intangibles or any other property) (hereinafter
collectively referred to as the "Collateral"):

            (a)   the Pledged Financial Assets and the certificates, if any,
representing the Pledged Financial Assets, and all dividends, interest, money
(as defined in the UCC (for the avoidance of doubt)), instruments and other
property from time to time received, receivable or otherwise distributed or
distributable in respect of or in exchange for any or all of such Pledged
Financial Assets;

            (b)   the Pledged Account and all security entitlements with respect
thereto, all Pledged Security Entitlements with respect to all Pledged Financial
Assets from time to time credited, or required to be credited to the Pledged
Account, any and all securities accounts in which the Pledged Security
Entitlements are carried, and all dividends, interest, cash, instruments and
other property from time to time received, receivable or otherwise distributed
or distributable in respect of or in exchange for any or all of such Pledged
Security Entitlements;

            (c)   all other securities, securities entitlements and other
financial assets hereafter acquired by the Pledgor pursuant to Article 12 of the
Indenture; and

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            (d)   all proceeds of any and all of the Collateral (including,
without limitation, proceeds that constitute property of the types described in
clauses (a), (b) and (c) of this Section 1), all investments of the Collateral
and, to the extent not otherwise included, all payments in respect of insurance
(whether or not the Trustee is the loss payee thereof) payable by reasons of
loss or damage or any indemnity, warranty or guaranty, or otherwise and all cash
proceeds, in each case with respect to the Collateral.

            SECTION 2. Security for Obligations. This Pledge Agreement secures,
and the Collateral is collateral security for, (x) the prompt and complete
payment and performance when due of the full amount of each of the first six
scheduled payments of interest on the Notes and (y) for the period beginning on
the date of this Pledge Agreement and ending on the scheduled date for the sixth
scheduled payment of interest on the Notes, the prompt and complete payment and
performance when due (whether at stated maturity, by acceleration or otherwise)
of all the Obligations of the Pledgor, whether for principal, interest,
Liquidated Damages, fees or otherwise, now or hereafter existing, under this
Pledge Agreement, the Notes or the Indenture (all such obligations being the
"Secured Obligations").

            SECTION 3. Maintaining the Pledged Account. So long as any Secured
Obligation shall remain outstanding:

            (a) The Pledgor will maintain separately the Pledged Account with
the Account Holder.

            (b) It shall be a term and condition of the Pledge Agreement,
notwithstanding any term or condition to the contrary in any other agreement
relating to the Pledged Account, and except as otherwise provided by Section 5
and Section 18 hereof, that no funds shall be paid or released to or for the
account of, or withdrawn by or for the account of, the Pledgor or any other
Person from the Pledged Account except as expressly provided in this Pledge
Agreement.

            The Pledged Account shall be subject to such applicable laws, and
such applicable regulations of the Board of Governors of the Federal Reserve
System and of any other appropriate banking or governmental authority, as may
now or hereafter be in effect.

            SECTION 4. Delivery of Collateral. (a) All certificates or
instruments representing or evidencing the Pledged Financial Assets or the
Pledged Security Entitlements shall be delivered to and held by or on behalf of
the Trustee pursuant hereto and shall be in suitable form for transfer by
delivery, or shall be accompanied by duly executed instruments of transfer or
assignment in blank, all in form and substance satisfactory to the Trustee. The
Trustee shall have the right, at any time in its discretion and without notice
to the Pledgor, to transfer to or to register in the name of the Trustee or any
of its nominees any or all of the Collateral. In addition, the Trustee shall
have the right at any time to exchange certificates or instruments representing
or evidencing any or all of the Collateral for certificates or instruments of
smaller or larger denominations. Also, the Trustee shall have the right at any
time to convert Collateral consisting of financial assets credited to the
Pledged Account to Collateral consisting of financial assets held directly by
the Trustee, and to convert Collateral consisting of financial assets held
directly by the Trustee to Collateral consisting of financial assets credited to
the Pledged Account.

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            (a) With respect to any Collateral in which the Pledgor has any
right, title or interest and that constitutes an uncertificated security, the
Pledgor shall cause the issuer thereof either (i) to register the Trustee as the
registered owner of such security or (ii) to agree in writing with the Pledgor
and the Trustee that such issuer will comply with instructions with respect to
such security originated by the Trustee without further consent of the Pledgor,
such agreement to be in form and substance satisfactory to the Trustee.

            (b) With respect to any Collateral in which the Pledgor has any
right, title or interest and that constitutes a security entitlement, the
Pledgor shall cause the securities intermediary with respect to such security
entitlement either (i) to identify in its records the Trustee as the entitlement
holder of such security entitlement against such securities intermediary or (ii)
to agree in writing with the Pledgor and the Trustee that such securities
intermediary will comply with entitlement orders (that is, notifications
communicated to such securities intermediary directing transfer or redemption of
the financial asset to which the Pledgor has a security entitlement) originated
by the Trustee without further consent of the Pledgor, such agreement to be in
substantially the form of Annex A hereto or otherwise in form and substance
satisfactory to the Trustee.

            (c) With respect to any Collateral that constitutes a securities
account, the Pledgor will comply with subsection (c) of this Section 4 with
respect to all security entitlements held in such securities account.

            (d) Prior to or concurrently with the execution and delivery hereof
and prior to the transfer to the Trustee of the Pledged Security Entitlements,
as provided in subsections (a) through (c) of this Section 4, the Trustee shall
establish the Pledged Account with the Account Holder. Upon transfer of the
Pledged Financial Assets to the Trustee, as confirmed to the Trustee by the
securities intermediary, the Trustee shall make appropriate book entries
indicating that the Pledged Financial Assets have been credited to and are held
in the Pledged Account. Subject to the other terms and conditions of this Pledge
Agreement, all funds or other property held by the Trustee pursuant to this
Pledge Agreement shall be held in the Pledged Account subject (except as
expressly provided in Sections 5(a), (b) and (c) hereof) to the exclusive
control (as defined in Section 8-106 of the UCC) of the Trustee and exclusively
for the benefit of the Trustee and for the ratable benefit of the Holders of the
Notes and segregated from all other funds or other property otherwise held by
the Trustee.

            (e) All Collateral shall be retained in the appropriate account
pending disbursement pursuant to the terms hereof.

            (f) Concurrently with the execution and delivery of this Pledge
Agreement, the Trustee shall deliver to the Pledgor a duly executed Control
Agreement (the "Control Agreement"), in the form of Annex A hereto.

            (g) Concurrently with the execution and delivery of this Pledge
Agreement, the Trustee shall deliver to the Pledgor a duly executed Control
Agreement (the "Control Agreement"), in the form of Annex A hereto.

            SECTION 5. Disbursements. (a) Three business days prior to the due
date of any of the first six scheduled interest payments on the Notes, the
Pledgor may, pursuant to written instructions given by the Pledgor to the
Trustee (an "Issuer Order"), direct the Trustee to release from the Pledged
Account and pay to the Holders of the Notes proceeds to provide for payment, in
whole or in part, of such interest then due on the Notes. Upon receipt of an
Issuer

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Order, the Trustee will (i) issue a Payment Order (as defined in the Control
Agreement) to the Account Holder for the release from the Pledged Account of
such funds to the Trustee in accordance with such Issuer Order and (ii) pay such
funds to the Holders of the Notes in accordance with the Indenture and the
Notes. In the event that the funds released from the Pledged Account on any
interest payment date are not sufficient to pay in full the interest due on the
Notes on such interest payment date (a "Shortfall"), nothing in this Section 5
shall relieve the Pledgor of its obligations under the Notes and the Indenture
to pay such Shortfall to the Holders of the Notes on such interest payment date;
provided, however, that no such Shortfall shall be paid from proceeds of the
Collateral unless such proceeds constitute a Surplus (as defined below).
Furthermore, nothing in this Pledge Agreement shall relieve the Pledgor of its
obligations under the Notes and the Indenture to pay Liquidated Damages, if any.
In the event that the funds available for release from the Pledged Account on
any interest payment date exceed the interest due on the Notes on such interest
payment date (a "Surplus"), such Surplus shall be retained in the Pledged
Account and may be applied to the next Shortfall(s), if any. Nothing in this
Section 5 shall affect the Trustee's rights to apply the Collateral to the
payments of amounts due on the Notes upon acceleration thereof.

            (b) If the Pledgor makes any of the first six scheduled interest
payments on the Notes or portion of such an interest payment from a source of
funds other than the Pledged Account ("Pledgor Funds"), the Pledgor may, after
payment in full of such interest payment, direct the Trustee pursuant to an
Issuer Order to issue a Payment Order (as defined in the Control Agreement) to
the Account Holder for the release to the Pledgor or to another party at the
direction of the Pledgor (the "Pledgor's Designee") proceeds from the Pledged
Account in an amount less than or equal to the amount of Pledgor Funds applied
to such interest payment; provided, however, that the foregoing shall not apply
in the event that such payment of Pledgor Funds is made in order to compensate
for any Shortfall. Upon receipt by the Trustee of such Issuer Order and provided
the Trustee has received such interest payment, the Trustee shall direct the
Account Holder pursuant to a Payment Order to pay over to the Pledgor or the
Pledgor's Designee, as the case may be, the requested amount from proceeds in
the Pledged Account as soon as practicable.

(c) At least three Business Days prior to the due date of each of the first six
scheduled interest payments on the Notes, the Pledgor shall give the Trustee
notice (by Issuer Order) as to whether such interest payment will be made
pursuant to Section 5(a) or 5(b) above and the respective amounts of interest
that will be paid from the Pledged Account and from Pledgor Funds. Any Pledgor
Funds to be used to make any interest payment shall be delivered to the Trustee,
in immediately available funds, prior to 10:00 a.m. (New York City time) on such
interest payment date. If no such notice is given or such Pledgor Funds have not
been so delivered, the Trustee will act pursuant to Section 5(a) above as if it
had received an Issuer Order pursuant thereto for the payment of the interest
then due from the Pledged Account.

(d) The Trustee shall instruct the Account Holder to liquidate Collateral in the
Pledged Account (pursuant to written instructions from the Pledgor) in order to
make any of the scheduled payments of interest on the Notes, unless there are
sufficient funds in the Pledged Account on such interest payment date. The
Trustee shall be entitled to instruct the Account Holder to sell any Collateral
as contemplated hereunder prior to the maturity of such Collateral and shall not
be responsible for any costs and expenses of such sale.

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            (e)   Nothing contained in this Pledge Agreement shall (i) afford
the Pledgor any right to issue entitlement orders with respect to any of the
Pledged Security Entitlements or any securities account in which any such
security entitlement may be carried, or otherwise afford the Pledgor control of
any Pledged Security Entitlement or (ii) otherwise give rise to any rights of
the Pledgor with respect to the Pledged Financial Assets or any securities
account in which any such security entitlement may be carried, other than the
Pledgor's rights under this Pledge Agreement as the beneficial owner of
collateral pledged to and subject to the exclusive control (except as expressly
provided in Sections 5(a) and (b) hereof prior to an Event of Default) of the
Trustee in its capacity as such (and not as a securities intermediary) before
the payment in full, when due, of the first six scheduled interest payments on
the Notes. The Pledgor acknowledges, confirms and agrees that the Trustee is an
entitlement holder of the Pledged Security Entitlements solely as Trustee for
the Holders of the Notes and not as a securities intermediary.

            SECTION 6. Investing of Amounts in the Pledged Account. If requested
and as directed in writing by the Pledgor, the Trustee will, subject to the
provisions of Sections 3, 5 and 13 of this Pledge Agreement, from time to time,
instruct the Account Holder to invest interest paid on the Pledged Financial
Assets and reinvest other proceeds of any Pledged Financial Assets that may
mature or be sold, in each case, in (i) identified United States Treasury
securities or (ii) selected shares of a money market fund registered under the
Investment Company Act of 1940, as amended, the portfolio of which consists of
United States Treasury securities, in each case credited to the Pledged Account.
For tax reporting and withholding purposes, all income earned on investments
held by the Trustee shall be allocated to the Pledgor and, within ten days after
the date of this Pledge Agreement, the Pledgor shall provide such tax reporting
documents to the Trustee as the Trustee may request.

            SECTION 7. Representations and Warranties. The Pledgor hereby
represents and warrants that:

            (a)   This Pledge Agreement has been duly authorized, validly
executed and delivered by the Pledgor and (assuming the due authorization and
valid execution and delivery of this Pledge Agreement by the Trustee and
enforceability of the Pledge Agreement against the Trustee in accordance with
its terms) constitutes a valid and binding agreement of the Pledgor, enforceable
against the Pledgor in accordance with its terms, except as (i) the
enforceability hereof may be limited by bankruptcy, insolvency, fraudulent
conveyance, preference, reorganization, moratorium or similar laws now or
hereafter in effect relating to or affecting creditors' rights or remedies
generally, (ii) the availability of equitable remedies may be limited by
equitable principles of general applicability, (iii) the exculpation provisions
and rights to indemnification hereunder may be limited by U.S. federal and state
securities laws and public policy considerations and (iv) the waiver of rights
and defenses contained in Section 13(d), Section 19(h) and Section 19(l) hereof
may be limited by applicable law.

            (b)   As of the date hereof and each Date of Delivery, (i) the
Pledgor's exact legal name (as defined in Section 9-503(a) of the UCC) is
Thoratec Corporation and (ii) the Pledgor is located (within the meaning of
Section 9-307 of the UCC) in the State of California and has its chief executive
office in the State of California.

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            (c)   The Pledgor is the legal and beneficial owner of the
Collateral free and clear of any Lien, claim, option or right of others (except
for the security interests created by this Pledge Agreement and any Liens
permitted under the Indenture). No effective financing statement or instrument
similar in effect covering all or any part of the Collateral or listing the
Pledgor or any trade name of the Pledgor with respect to all or any part of the
Collateral is on file in any public or recording office, other than the
financing statements filed pursuant to this Pledge Agreement.

            (d)   All filings and other actions (including, without limitation,
(A) actions necessary to obtain control of the Collateral as provided in
Sections 9-104, 9-105, 9-106 and 9-107 of the UCC and (B) actions necessary to
perfect the Trustee's security interest with respect to the Collateral evidenced
by a certificate of ownership) necessary to perfect the security interest in the
Collateral created under this Pledge Agreement have been duly made or taken and
are in full force and effect, and this Pledge Agreement creates in favor of the
Trustee for its benefit and the ratable benefit of the Holders of the Notes a
valid and, together with such filings and other actions, a perfected first
priority security interest in the Collateral, securing the payment of the
Secured Obligations.

            (e)   The execution and delivery by the Pledgor of, and the
performance by the Pledgor of its obligations under, this Pledge Agreement will
not contravene any provision of applicable law or the Articles of Incorporation
of the Pledgor or any material agreement or other material instrument binding
upon the Pledgor or any judgment, order or decree of any governmental body,
agency or court having jurisdiction over the Pledgor, or result in the creation
or imposition of any Lien on any assets of the Pledgor, except for the security
interests granted under this Pledge Agreement.

            (f)   No consent of any other person and no approval, authorization,
order of, action by, notice to, filing or qualification with, any governmental
authority, regulatory body, agency or other third party is required for (i) the
grant by the Pledgor of the pledge and security interest granted under this
Pledge Agreement, (ii) the execution or delivery by the Pledgor of, or the
performance by the Pledgor of its obligations under, this Pledge Agreement,
(iii) the perfection or maintenance of the pledge and security interest created
hereunder (including the first priority nature of such pledge or security
interest), except for the filing of financing and continuation statements under
the UCC, which financing statements have been delivered to the Trustee and
which, when filed, shall be in full force and effect, or (iv) for the exercise
by the Trustee of its voting or other rights provided for in this Pledge
Agreement or the remedies in respect of the Collateral pursuant to this Pledge
Agreement, except as may be required in connection with the disposition of any
portion of the Collateral by laws affecting the offering and sale of securities
generally.

            (g)   As of the date hereof and each Date of Delivery, there are no
legal or governmental proceedings pending or, to the Pledgor's knowledge,
threatened to which the Pledgor is a party or to which any of the properties of
the Pledgor is subject that would materially adversely affect the power or
ability of the Pledgor to perform its obligations under this Pledge Agreement or
to consummate the transactions contemplated hereby.

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            (h)   The pledge of the Collateral pursuant to this Pledge Agreement
is not prohibited by law or governmental regulation (including, without
limitation, Regulations T, U and X of the Board of Governors of the Federal
Reserve System) applicable to the Pledgor.

            (i)   As of the date hereof and each Date of Delivery, no Event of
Default (as defined below) exists.

            SECTION 8. Further Assurances. (a) The Pledgor agrees that from time
to time, at the expense of the Pledgor, the Pledgor will promptly execute and
deliver all further instruments and documents, and take all further action, that
may be necessary or desirable, and that the Trustee may reasonably request, in
order to perfect and protect any pledge or security interest granted or
purported to be granted hereunder or to enable the Trustee to exercise and
enforce its rights and remedies hereunder with respect to any Collateral.
Without limiting the generality of the foregoing, the Pledgor will: (i) if any
Collateral shall be evidenced by a promissory note or other instrument, deliver
and pledge to the Trustee hereunder such note or instrument, duly endorsed and
accompanied by duly executed instruments of transfer or assignment, all in form
and substance satisfactory to the Trustee; (ii) execute and file such financing
or continuation statements, or amendments thereto, and such other instruments or
notices, as may be necessary or desirable, or as the Trustee may reasonably
request, in order to perfect and preserve the pledge and security interest
granted or purported to be granted hereby; (iii) deliver and pledge to the
Trustee for its benefit and the ratable benefit of the Holders of the Notes
certificates representing Collateral that constitutes certificated securities,
accompanied by undated stock or bond powers executed in blank; and (iv) deliver
to the Trustee evidence that all other action that the Trustee may deem
reasonably necessary or desirable in order to perfect and protect the security
interest created by the Pledgor under this Pledge Agreement has been taken.

            (b)   The Pledgor hereby authorizes the Trustee to file one or more
financing or continuation statements, and amendments thereto, with respect to
the Collateral, in each case without the signature of the Pledgor, and
regardless of whether any particular asset described in such financing
statements falls within the scope of the UCC. Such financing statements may
describe the Collateral by reference to an attached photocopy or other
reproduction of this Pledge Agreement, or in any manner similar in scope to the
description of the Collateral provided in Section 1. The Pledgor ratifies its
authorization for the Trustee to have filed such financing statements,
continuation statements or amendments filed prior to the date hereof.

            (c)   The Pledgor will furnish to the Trustee from time to time
statements and schedules further identifying and describing the Collateral and
such other reports in connection with the Collateral as the Trustee may
reasonably request, all in reasonable detail.

            (d)   The Pledgor will promptly pay all reasonable costs and
expenses incurred in connection with any of the foregoing within 45 days after
receipt of an invoice therefor. The Pledgor also agrees, promptly upon request
by the Trustee, to take all actions that are necessary to perfect or continue
the perfection of, or to protect the first priority of, the Trustee's security
interest in and to the Collateral, including the filing of all necessary
financing and continuation statements, and to protect the Collateral against the
rights, claims or interests of third persons (other than any such rights, claims
or interests created by or arising through the Trustee).

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            SECTION 9. Covenants. The Pledgor covenants and agrees with the
Trustee and the Holders of the Notes that, from and after the date of this
Pledge Agreement until the earlier to occur of (x) payment in full in cash of
each of the first six scheduled interest payments on the Notes when due under
the terms of the Indenture and (y) payment in full of all obligations due and
owing under the Indenture and the Notes in the event such obligations become due
and payable prior to the scheduled date for payment of the sixth scheduled
interest payment on the Notes:

            (a)   except as permitted under the Indenture, (A) it will not (and
will not purport to) sell, assign or otherwise dispose of, or grant any option
or warrant with respect to, any of the Collateral or its beneficial interest
therein, and (B) it will not create or suffer to exist any Lien or other adverse
interest upon or with respect to any of the Collateral or its beneficial
interest therein (except for the security interests granted under this Pledge
Agreement);

            (b)   except as permitted under the Indenture, it will not (A) enter
into any agreement or understanding that restricts or inhibits or purports to
restrict or inhibit the Trustee's rights or remedies hereunder, including,
without limitation, the Trustee's right to sell or otherwise dispose of the
Collateral or (B) fail to pay or discharge any tax, assessment or levy of any
nature with respect to its beneficial interest in the Collateral not later than
five days prior to the date of any proposed sale under any judgment, writ or
warrant of attachment with respect to such beneficial interest;

            (c)   it will not change its name, type of organization,
jurisdiction of organization, organizational identification number or location
from those set forth in Section 7(b) hereof without first giving at least 10
days' prior written notice to the Trustee and taking all action required by the
Trustee for the purpose of perfecting or protecting the security interest
granted by this Pledge Agreement; and

            (d)   it will, and will cause the Trustee and the Account Holder to,
execute and deliver on or prior to any Date of Delivery (as defined in Section
2(b) of the Purchase Agreement) a supplement to this Pledge Agreement,
reasonably satisfactory to the Initial Purchaser, providing for the pledge of
additional Collateral to secure all Obligations in respect of the Option Notes.

            SECTION 10. Power of Attorney. In addition to all of the powers
granted to the Trustee pursuant to the Indenture, the Pledgor hereby irrevocably
appoints the Trustee as the Pledgor's attorney-in-fact (with full power of
substitution), with full authority in the place and stead of the Pledgor and in
the name of the Pledgor or otherwise, from time to time in the Trustee's
discretion after the occurrence and during the continuance of an Event of
Default, to take any action and to execute any instrument that is necessary or
advisable or as the Trustee may deem necessary or advisable to accomplish the
purposes of this Pledge Agreement, including, without limitation, to:

            (a)   ask for, demand, collect, sue for, recover, compromise,
receive and give acquittance and receipts for moneys due and to become due under
or in respect of any of the Collateral,

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            (b)   receive, indorse and collect any drafts or other instruments,
documents and chattel paper, in connection with clause (a) above,

            (c)   file any claims or take any action or institute any
proceedings that the Trustee may deem necessary or desirable for the collection
of any of the Collateral or otherwise to enforce the rights of the Trustee with
respect to any of the Collateral, and

            (d)   pay or discharge taxes or Liens levied or placed upon the
Collateral that the Pledgor has failed to pay or discharge in accordance
herewith, the legality or validity thereof and the amounts necessary to
discharge the same to be determined by the Trustee in its sole reasonable
discretion, and such payments made by the Trustee to become part of the
Obligations of the Pledgor to the Trustee, due and payable immediately upon
demand;

provided, however, that the Trustee shall have no obligation to perform any of
the foregoing actions. The Trustee's authority under this Section 10 shall
include, without limitation, the authority to endorse and negotiate any checks
or instruments representing proceeds of Collateral in the name of the Pledgor,
execute and give receipt for any certificate of ownership or any document
constituting Collateral, transfer title to any item of Collateral, or any other
documents deemed necessary or appropriate by the Trustee to preserve, protect or
perfect the security interest in the Collateral granted hereunder and to file
the same, prepare, file and sign the Pledgor's name on any notice of Lien, and
to take any other actions arising from or incident to the powers granted to the
Trustee in this Pledge Agreement. This power of attorney is coupled with an
interest and is irrevocable by the Pledgor.

            If the Pledgor fails to perform any agreement contained herein, the
Trustee may, but without obligation to do so and without notice, itself perform,
or cause performance of, such agreement, and the expenses of the Trustee
incurred in connection therewith shall be payable by the Pledgor under Section
12 hereof.

            SECTION 11. No Assumption of Duties; Reasonable Care. The powers
conferred on the Trustee hereunder are solely to protect the security interest
of the Trustee for its benefit and the ratable benefit of the Holders of the
Notes in the Collateral and shall not impose any duty on the Trustee to exercise
any such powers. Except for the safe custody of any Collateral in its possession
and the accounting for moneys actually received by it hereunder, the Trustee
shall have no duty as to any Collateral for (i) ascertaining or taking action
with respect to calls, conversions, exchanges, maturities, tenders or other
matters relative to any Collateral, whether or not the Trustee has or is deemed
to have knowledge of such matters, (ii) taking any necessary steps to preserve
rights against any parties or any other rights pertaining to any Collateral or
(c) investing or reinvesting any of the Collateral or any loss on any
investment. The Trustee shall be deemed to have exercised reasonable care in the
custody and preservation of any Collateral in its possession if such Collateral
is accorded treatment substantially equal to that which it accords its own
property. The Trustee shall be entitled to all the rights, benefits, privileges
and immunities accorded to it under the Indenture.

            SECTION 12. Indemnity and Expenses. (a) The Pledgor agrees to
indemnify, defend and save and hold harmless the Trustee, and each Holder of
Notes and their respective officers, directors, employees, agents and advisors
(each, an "Indemnified Party") from and

                                       10
<PAGE>

against, and shall pay on demand, any and all claims, damages, losses,
liabilities and expenses (including, without limitation, reasonable fees and
expenses of counsel) that may be incurred by or asserted or awarded against any
Indemnified Party, in each case arising out of or in connection with or
resulting from this Pledge Agreement (including, without limitation, enforcement
of this Pledge Agreement), except to the extent such claim, damage, loss,
liability or expense is from such Indemnified Party's gross negligence or
willful misconduct.

            (b)   The Pledgor will, within five business days after demand, pay
to the Trustee the amount of any and all reasonable fees and expenses,
including, without limitation, the reasonable fees and expenses of its counsel
and of any experts and agents, that the Trustee may incur in connection with (i)
the review and negotiation of the Indenture and this Pledge Agreement, (ii) the
custody or preservation of, or the sale of, collection from or other realization
upon, any of the Collateral, (iii) the exercise or enforcement of any of the
rights of the Trustee or the Holders of the Notes hereunder or (iv) the failure
by the Pledgor to perform or observe any of the provisions hereof.

            SECTION 13. Remedies. If any Event of Default under the Indenture
(including, without limitation, any default hereunder constituting an Event of
Default (any such Event of Default being referred to in this Pledge Agreement as
an "Event of Default")) shall have occurred and be continuing and payments on
the Notes shall have been accelerated in accordance with the provisions of the
Indenture:

            (a)   The Trustee and the Holders of the Notes may exercise in
respect of the Collateral, in addition to all other rights and remedies given by
law or by this Pledge Agreement or the Indenture, all of the rights and remedies
of a secured party under the UCC (whether or not the UCC applies to the affected
Collateral) and also may without notice except as specified below, sell the
Collateral or any part thereof in one or more parcels at any broker's board or
at public or private sale, in one or more sales or lots, at any of the Trustee's
offices or elsewhere, for cash, on credit or for future delivery, and upon such
other terms as the Trustee may deem commercially reasonable. The Pledgor agrees
that, to the extent notice of sale shall be required by law, at least 10 days'
notice to the Pledgor of the time and place of any public sale or the time after
which any private sale is to be made shall constitute reasonable notification.
The Trustee shall not be obligated to make any sale of Collateral regardless of
notice of sale having been given. The Trustee may adjourn any public or private
sale from time to time by announcement at the time and place fixed therefor, and
such sale may, without further notice, be made at the time and place to which it
was so adjourned. The purchaser of any or all Collateral so sold shall
thereafter hold the same absolutely, free from any claim, encumbrance or right
of any kind whatsoever created by or through the Pledgor. Any sale of the
Collateral conducted in conformity with reasonable commercial practices of
banks, insurance companies, commercial finance companies, or other financial
institutions disposing of property similar to the Collateral shall be deemed to
be commercially reasonable. The Trustee or any Holder of Notes may, in its own
name or in the name of a designee or nominee, buy any of the Collateral at any
public sale and, if permitted by applicable law, at any private sale. All
expenses (including court costs and reasonable attorneys' fees, expenses and
disbursements) of, or incident to, the enforcement of any of the provisions
hereof shall be recoverable from the proceeds of the sale or other disposition
of the Collateral.

                                       11
<PAGE>

            (b)   Any cash held by or on behalf of the Trustee and all cash
proceeds received by or on behalf of the Trustee in respect of any sale of,
collection from, or other realization upon all or any part of the Collateral
shall (after payment of any amounts payable to the Trustee pursuant to Section
12(b) of this Pledge Agreement) be promptly released by the Trustee for payment
to the Holders of the Notes and be applied first to any accrued and unpaid
interest on the Notes, second, to the repayment of the principal amount of the
Notes and Liquidated Damages (if any) on the Notes, and third, to the extent
available, against the remaining Secured Obligations. Any surplus of such cash
or cash proceeds held by or on behalf of the Trustee and remaining after payment
in full of all the Secured Obligations shall be paid over to the Pledgor or to
whomsoever may be lawfully entitled to receive such surplus.

            (c)   The Trustee may, without notice to the Pledgor except as
required by law and at any time or from time to time, charge, set-off and
otherwise apply all or any part of the Collateral against the Secured
Obligations or any part thereof.

            (d)   The Pledgor agrees to (i) provide the Trustee with such
information as may be necessary, or in the opinion of the Trustee, advisable to
enable the Trustee to effect the sale of the Collateral and (ii) use its
reasonable best efforts to do or cause to be done all such other acts and things
as may be necessary to make such sale or sales of all or any portion of the
Collateral pursuant to this Section 13 valid and binding and in compliance with
any and all other applicable requirements of law.

            SECTION 14. Security Interest Absolute. This Pledge Agreement shall
continue to be effective or be reinstated, as the case may be, if at any time
any payment of any of the Secured Obligations is rescinded or must otherwise be
returned by the Trustee or any Holder of the Notes or by any other Person upon
the insolvency, bankruptcy or reorganization of the Pledgor or otherwise, all as
though such payment had not been made.

            SECTION 15. Amendments, Waivers and Consents. No amendment or waiver
of any provision of this Pledge Agreement, and no consent to any departure by
the Pledgor from any provision of this Pledge Agreement, shall in any event be
effective unless the same shall be in writing and signed by the Trustee and the
Pledgor, and then such waiver or consent shall be effective only in the specific
instance and for the specific purpose for which given. No failure on the part of
the Trustee or any Holder of the Notes to exercise, and no delay in exercising
any right hereunder, shall operate as a waiver thereof; nor shall any single or
partial exercise of any such right preclude any other or further exercise
thereof or the exercise of any other right.

            SECTION 16. Notices. Any notice or communication given hereunder
shall be sufficiently given if in writing and delivered in person or mailed by
first class mail, commercial courier service or telecopier communication,
addressed as follows; or, as to any party, at such other address as shall be
designated by such party in a written notice to the other parties:

                                       12
<PAGE>

                  if to the Pledgor:

                        Thoratec Corporation
                        6035 Stoneridge Drive
                        Pleasanton, California 94588
                        Attention: General Counsel
                        Telephone: (925) 847-6000
                        Fax: (925) 847-8574

                        with a copy to:

                        Gibson, Dunn & Crutcher LLP
                        One Montgomery Street, Suite 3100
                        San Francisco, California 94104
                        Attention: Gregory J. Conklin
                        Telephone: (415) 393-8200
                        Fax: (415) 986-5309

                  if to the Trustee:

                        U.S. Bank National
                        Association 633 West Fifth
                        Street, 24th Floor Los
                        Angeles, California 90071
                        Attention: Paula Oswald
                        Telephone: (213) 615-6043
                        Fax: (213) 615-6197

All such notices and other communications shall, when mailed, delivered or
telecopied, respectively, be effective when deposited in the mails, delivered or
telecopied, respectively, addressed as aforesaid. Notwithstanding the foregoing,
notices to the Trustee shall be effective only upon receipt.

            SECTION 17. Continuing Security Interest. This Pledge Agreement
shall create a continuing security interest in the Collateral and (a) shall,
unless otherwise provided in this Pledge Agreement, remain in full force and
effect until the payment in full in cash of the Secured Obligations, (b) be
binding upon the Pledgor, its successors and assigns and (c) inure, together
with the rights and remedies of the Trustee hereunder, to the benefit of the
Trustee and the Holders of the Notes and their respective successors,
transferees and assigns.

            SECTION 18. Termination. This Pledge Agreement (other than the
Pledgor's obligations under Section 12 hereof) shall terminate upon the earlier
of (i) the redemption of the Notes in whole, (ii) the payment in full of each of
the first six scheduled interest payments on the Notes when due, or (iii) the
discharge of the Indenture. Upon any such termination, without any necessary
action on the part of the Pledgor or the Trustee or Account Holder, (i) the
Control Agreement(s) will terminate and control of the Pledged Account and the
Pledged Security Entitlements shall revert to the Pledgor, (ii) the Trustee
shall promptly notify the Account Holder and obtain from the Account Holder and
deliver to the Pledgor all certificates and instruments

                                       13
<PAGE>

representing any portion of the Pledged Financial Assets constituting
certificated securities and (iii) the Trustee shall no longer have any rights in
any of the Collateral.

            SECTION 19. Miscellaneous Provisions.

            (a)   No Adverse Interpretation of Other Agreements. This Pledge
Agreement may not be used to interpret another pledge, security or debt
agreement of the Pledgor or any subsidiary thereof. No such pledge, security or
debt agreement (other than the Indenture) may be used to interpret this Pledge
Agreement.

            (b)   Severability. The provisions of this Pledge Agreement are
severable, and if any clause or provision shall be held invalid, illegal or
unenforceable in whole or in part in any jurisdiction, then such invalidity or
unenforceability shall affect in that jurisdiction only such clause or
provision, or part thereof, and shall not in any manner affect such clause or
provision in any other jurisdiction or any other clause or provision of this
Pledge Agreement in any jurisdiction.

            (c)   Headings. The headings in this Pledge Agreement have been
inserted for convenience of reference only, are not to be considered a part
hereof and shall in no way modify or restrict any of the terms or provisions
hereof.

            (d)   Counterpart Originals. This Pledge Agreement may be signed by
facsimile and in two or more counterparts, each of which shall be deemed an
original, but all of which shall together constitute one and the same agreement.
Delivery of an executed counterpart of a signature page to this Pledge Agreement
by telecopier shall be effective as delivery of an original executed counterpart
of this Pledge Agreement.

            (e)   Benefits of Pledge Agreement. Nothing in this Pledge
Agreement, express or implied, shall give to any person, other than the parties
hereto and their successors hereunder, and the Holders of the Notes and the
Account Holder, any benefit or any legal or equitable right, remedy or claim
under this Pledge Agreement.

            (f)   Interpretation of Agreement. To the extent a term or provision
of this Pledge Agreement conflicts with the Indenture, the Indenture shall
control with respect to the subject matter of such term or provision. Acceptance
of or acquiescence in a course of performance rendered under this Pledge
Agreement shall not be relevant to determine the meaning of this Pledge
Agreement even though the accepting or acquiescing party had knowledge of the
nature of the performance and opportunity for objection.

            (g)   Survival of Representations and Covenants. All
representations, warranties and covenants of the Pledgor contained herein shall
survive the execution and delivery of this Pledge Agreement, and shall terminate
only upon the termination of this Pledge Agreement, except as otherwise
specified in such representations, warranties and covenants.

            (h)   Waivers. The Pledgor waives presentment and demand for payment
of any of the Obligations, protest and notice of dishonor or default with
respect to any of the Obligations, and all other notices to which the Pledgor
might otherwise be entitled, except as otherwise expressly provided herein or in
the Indenture.

                                       14
<PAGE>

            (i)   Authority of the Trustee. (i) The Trustee shall have and be
entitled to exercise all powers hereunder that are specifically granted to the
Trustee by the terms hereof, together with such powers as are reasonably
incident thereto. The Trustee may perform any of its duties hereunder or in
connection with the Collateral by or through agents or employees and shall be
entitled to retain counsel and to act in reliance upon the advice of counsel
concerning all such matters. Except as otherwise expressly provided in this
Pledge Agreement or the Indenture, neither the Trustee nor any director,
officer, employee, attorney or agent of the Trustee shall be liable to the
Pledgor for any action taken or omitted to be taken by the Trustee, in its
capacity as Trustee, hereunder, except for its own bad faith, gross negligence
or willful misconduct, and the Trustee shall not be responsible for the
validity, effectiveness or sufficiency hereof or of any document or security
furnished pursuant hereto. The Trustee and its directors, officers, employees,
attorneys and agents shall be entitled to rely on any communication, instrument
or document believed by it or them to be genuine and correct and to have been
signed or sent by the proper person or persons.

                  (ii)  The Pledgor acknowledges that the rights and
responsibilities of the Trustee under this Pledge Agreement with respect to any
action taken by the Trustee or the exercise or non-exercise by the Trustee of
any option, right, request, judgment or other right or remedy provided for
herein or resulting or arising out of this Pledge Agreement shall, as between
the Trustee and the Holders of the Notes, be governed by the Indenture and by
such other agreements with respect thereto as may exist from time to time among
them, but, as between the Trustee and the Pledgor, the Trustee shall be
conclusively presumed to be acting as agent for the Holders of the Notes with
full and valid authority so to act or refrain from acting, and the Pledgor shall
not be obligated or entitled to make any inquiry respecting such authority.

            (j)   Final Expression. This Pledge Agreement, together with the
Indenture and any other agreement executed in connection herewith, is intended
by the parties as a final expression of this Pledge Agreement and is intended as
a complete and exclusive statement of the terms and conditions thereof.

            (k)   Rights of Holders of the Notes. No Holder of Notes shall have
any independent rights hereunder other than those rights granted to individual
Holders of the Notes pursuant to Section 6.06 of the Indenture; provided that
nothing in this subsection shall limit any rights granted to the Trustee under
the Notes or the Indenture.

            (l)   Governing Law; Submission to Jurisdiction; Waiver of Jury
Trial; Waiver of Damages. (i) This Pledge Agreement shall be governed by, and
construed in accordance with, the laws of the State of New York.

                  (ii)  The Pledgor agrees that the Trustee shall, in its
capacity as trustee or in the name and on behalf of any Holder of Notes, have
the right, to the extent permitted by applicable law, to proceed against the
Pledgor or the Collateral in a court in any location reasonably selected in good
faith (and having personal or in rem jurisdiction over the Pledgor or the
Collateral, as the case may be) to enable the Trustee to realize on the
Collateral, or to enforce a judgment or other court order entered in favor of
the Trustee. The Pledgor agrees that it will not assert any counterclaims,
setoffs or crossclaims in any proceeding brought by the Trustee to realize on
such property or to enforce a judgment or other court order in favor of the
Trustee,

                                       15
<PAGE>

except for such counterclaims, setoffs or crossclaims which, if not asserted in
any such proceeding, could not otherwise be brought or asserted. The Pledgor
waives any objection that it may have to the location of the court in the City
of New York once the Trustee has commenced a proceeding described in this
paragraph including, without limitation, any objection to the laying of venue or
based on the grounds of forum non conveniens.

                  (iii)  The Pledgor agrees that neither any Holder of Notes nor
(except as otherwise provided in this Pledge Agreement or the Indenture) the
Trustee in its capacity as trustee shall have any liability to the Pledgor
(whether arising in tort, contract or otherwise) for losses suffered by the
Pledgor in connection with, arising out of, or in any way related to, the
transactions contemplated and the relationship established by this Pledge
Agreement, or any act, omission or event occurring in connection therewith,
unless such losses were the result of acts or omissions on the part of the
Trustee or such Holder of Notes, as the case may be, constituting bad faith,
gross negligence or willful misconduct.

                  (iv)  To the extent permitted by applicable law, the Pledgor
waives the posting of any bond otherwise required of the Trustee or any Holder
of Notes in connection with any judicial process or proceeding to enforce any
judgment or other court order pertaining to this Pledge Agreement or any related
agreement or document entered in favor of the Trustee or any Holder of Notes, or
to enforce by specific performance, temporary restraining order or preliminary
or permanent injunction, this Pledge Agreement or any related agreement or
document between the Pledgor on the one hand and the Trustee and/or the Holders
of the Notes on the other hand.

                                       16
<PAGE>

            IN WITNESS WHEREOF, the Pledgor and the Trustee have each caused
this Pledge Agreement to be duly executed and delivered as of the date first
above written.

                                   Pledgor:

                                   THORATEC CORPORATION

                                   By: /s/ D. Keith Grossman
                                       -----------------------------------------
                                   Name: D. Keith Grossman
                                   Title: President and Chief Executive Officer

                    [Signature page to the Pledge Agreement]
<PAGE>

                                   IN WITNESS WHEREOF, the Pledgor and the
                                   Trustee have each caused this Pledge
                                   Agreement to be duly executed and
                                   delivered as of the date first above
                                   written.

                                   Trustee:

                                   U.S. BANK NATIONAL ASSOCIATION

                                   By: /s/ Paula M. Oswald
                                       -----------------------------------------
                                       Name: Paula M. Oswald
                                       Title: Vice President

                    [Signature page to the Pledge Agreement]
<PAGE>

                                   SCHEDULE I

                            Pledged Financial Assets
<TABLE>
<CAPTION>
Par Amount                 Maturity Date           CUSIP No.
----------              ------------------         ---------
<S>                      <C>                       <C>
1,419,000                November 15, 2004         912833FV7

1,485,000                   May 15, 2005           912833FW5

1,485,000                November 15, 2005         912833FX3

1,485,000                   May 15, 2006           912833FY1

1,485,000                November 15, 2006         912833FZ8

1,485,000                   May 15, 2007           912833GA2
</TABLE>

                                     Sch - 1
<PAGE>

                                                                         ANNEX A

                                CONTROL AGREEMENT

            This CONTROL AGREEMENT (this "Agreement") dated as of May 24, 2004
by and among Thoratec Corporation (the "Pledgor") and U.S. Bank National
Association, in its capacity as trustee (the "Trustee"), and U.S. Bank National
Association, a national banking association, in its capacity as securities
intermediary and depository bank (the "Account Holder").

                             PRELIMINARY STATEMENTS:

            (1) The Pledgor has granted the Trustee a security interest (the
"Security Interest") in certain security entitlements (the "Pledged Security
Entitlements") with respect to certain U.S. Treasury securities (the "Pledged
Financial Assets") identified on Schedule I attached hereto maintained by the
Trustee with the Account Holder and carried from time to time in an account with
the Account Holder, ABA No. 091000022, Account No. 785834000, and entitled
"Thoratec Corporation Senior Subordinated Convertible Notes due 2034, Collateral
Pledge Account" (the "Pledged Account") and all additions thereto and
substitutions and proceeds thereof (collectively, the "Collateral"), pursuant
to, and as more particularly described in, a Pledge Agreement dated as of May
18, 2004, among the Pledgor and the Trustee (as the same may hereafter be
amended, supplemented or otherwise modified from time to time, the "Pledge
Agreement"; terms defined in the Pledge Agreement and not otherwise defined
herein are used herein as therein defined). The Pledgor acknowledges having
received value for such pledge of the Collateral.

            (2) Terms defined in Article 8 or 9 of the Uniform Commercial Code
as in effect in the State of New York (the "UCC") are used in this Agreement
(including, without limitation, paragraph (1) above) as such terms are defined
in such Article 8 or 9.

            (3) The Pledgor, the Trustee and the Account Holder are delivering
this Agreement pursuant to the terms of the Pledge Agreement.

            NOW, THEREFORE, in consideration of the premises and mutual
agreements contained herein, the parties hereto hereby agree as follows:

            SECTION 1. Notice of Exclusive Control. The Pledgor and Trustee are
entering into this Agreement to perfect, and confirm the first priority lien of,
the Trustee's Security Interest in the Collateral. The Account Holder agrees to
promptly make all necessary entries or notations in its books and records to
reflect the Trustee's Security Interest in the Collateral and to apply any value
distributed on account of any Pledged Financial Assets as directed in writing by
the Trustee without further consent from the Pledgor. The Account Holder
acknowledges that the Trustee has exclusive control over the Pledged Account and
all Pledged Security Entitlements contained therein from time to time.

                                      A - 1
<PAGE>

            SECTION 2. The Account. The Account Holder represents and warrants
to, and agrees with, the Pledgor and the Trustee and the Holders of the Notes
that:

            (a) The Account Holder has established the Pledged Account and shall
not change the name or account number of the Pledged Account without the prior
written consent of the Trustee.

            (b) The Account Holder maintains the Pledged Account for the
Trustee, and all property (including, without limitation, all funds and
financial assets) held by the Account Holder for the account of the Trustee is,
and will continue to be, credited to the Pledged Account. The Account Holder is
a securities intermediary (as defined in Section 8-102(a)(xiv) of the UCC) and
is acting in such capacity in relation to the Trustee, the Pledgor and the
Pledged Account and with respect to all property credited thereto, from time to
time.

            (c) The Pledged Account is a securities account. The Account Holder
is the securities intermediary with respect to financial assets credited to the
Pledged Account from time to time. The Trustee is the entitlement holder with
respect to financial assets credited from time to time to the Pledged Account.

            (d) All financial assets in registered form or payable to or to the
order of and credited to the Pledged Account shall be registered in the name of,
payable to or to the order of, or endorsed to, the Account Holder and in no case
during the term of the Pledge Agreement will any financial asset credited to the
Pledged Account be registered in the name of, payable to or to the order of, or
endorsed to, the Pledgor, except to the extent the foregoing have been
subsequently endorsed by the Pledgor to the Account Holder or in blank.

            (e) Notwithstanding any other agreement to the contrary, the Account
Holder's jurisdiction with respect to the Pledged Account for purposes of the
UCC is, and will continue to be for so long as the Security Interest shall be in
effect, the State of New York.

            (f) The Account Holder does not know of any claim to or interest in
the Pledged Account or any property (including, without limitation, all funds
and financial assets) credited to the Pledged Account, except for claims and
interests of the parties referred to in this Agreement.

            SECTION 3. Control by Trustee. (a) The Account Holder will comply
with (A) all written instructions directing disposition of the funds in the
Pledged Account (such instructions, a "Payment Order"), (B) all notifications
and entitlement orders that the Account Holder receives directing it to transfer
or redeem any financial asset in the Pledged Account and (C) all other
directions concerning the Collateral, including, without limitation, directions
to distribute to the Trustee proceeds of any such transfer or redemption or
interest on any property in the Pledged Account (any such instruction,
notification or direction referred to in clause (A), (B) or (C) above being an
"Account Direction"), in each case of clauses (A), (B) and (C) above originated
by the Trustee without further consent by the Pledgor or any other person.

            (b) The Trustee hereby acknowledges that it shall maintain and
exercise control of the Pledged Account on behalf of the Holders of the Notes.

                                      A - 2
<PAGE>

            (c) The Account Holder will not (i) comply with Account Directions
or other directions concerning the Collateral originated by the Pledgor or (ii)
distribute to the Pledgor interest or other distributions on or in respect of
the Collateral, unless instructed to do so by the Trustee, in its absolute
discretion.

            SECTION 4. Priority of Trustee's Security Interest. (a) The Account
Holder (i) hereby subordinates to the Security Interest and in favor of the
Trustee any security interest, lien, or right of setoff the Account Holder may
have, now or in the future, against the Pledged Account or property in the
Pledged Account, and (ii) agrees that it will not exercise any right in respect
of any such security interest or lien or any such right of setoff until the
Security Interest is terminated, except that the Account Holder will retain its
prior lien on property in the Pledged Account to secure payment for property
purchased for the Pledged Account and normal commissions and fees for the
Pledged Account.

            (b) The Account Holder will not enter into any other agreement with
any Person relating to Account Directions or other directions with respect to
the Pledged Account.

            SECTION 5. Statements, Confirmations, and Notices of Adverse Claims.
(a) The Account Holder will send copies of all statements and confirmations for
the Pledged Account simultaneously to the Pledgor and the Trustee. So long as
the Trustee and the Account Holder are the same institution, the Account Holder
is not required to send copies of statements and confirmations as provided in
the preceding sentence.

            (b) When the Account Holder knows of any claim or interest in the
Pledged Account or any property credited to the Pledged Account other than the
claims and interests of the parties referred to in this Agreement, the Account
Holder will promptly notify the Trustee and the Pledgor of such claim or
interest.

            SECTION 6. The Account Holder's Responsibility. (a) The Account
Holder will not be liable to the Pledgor or the Trustee or the Holders of the
Notes for complying with an Account Direction or other direction concerning the
Collateral originated by the Trustee, even if the Pledgor notifies the Account
Holder that the Trustee is not legally entitled to issue the Account Direction
or such other direction unless the Account Holder takes the action after it is
served with an injunction, restraining order, or other legal process enjoining
it from doing so, issued by a court of competent jurisdiction, and had a
reasonable opportunity to act on the injunction, restraining order or other
legal process.

            (b) This Agreement does not create any obligation of the Account
Holder except for those expressly set forth in this Agreement, in Part 5 of
Article 8 of the UCC, or in Article 4 of the UCC. In particular, the Account
Holder need not investigate whether the Trustee is entitled under the Trustee's
agreements with the Pledgor to give an Account Direction or other direction
concerning the Pledged Account. The Account Holder may conclusively rely on
notices and communications it believes given by the appropriate party.

            (c) In no event shall the Account Holder or any of its affiliates,
shareholders, directors, officers, employees or agents be liable for indirect,
special, punitive, incidental or consequential damages of any kind whatsoever
even if advised of the possibility of such

                                     A - 3
<PAGE>

damages, other than such damages caused by its own bad faith, gross negligence
or willful misconduct.

            (d) Without limiting the foregoing, and notwithstanding any
provision to the contrary elsewhere, the Account Holder and its affiliates,
shareholders, directors, officers, employees or agents:

                  (i)   shall have no responsibilities, obligations or duties in
respect of the subject matter hereof other than those expressly set forth in
this Agreement, and no implied duties, responsibilities, covenants or
obligations shall be read into this Agreement against the Account Holder.
Without limiting the foregoing, the Account Holder shall have no duty or
authority to determine and/or investigate whether or not an event of default
exists under any agreement between the Pledgor and the Trustee, or to determine
and/or investigate whether or not the Trustee is entitled to give any Account
Direction with respect to the Collateral;

                  (ii)  may in any instance where the Account Holder determines
that it lacks or is uncertain as to its authority to take or refrain from taking
certain action hereunder, or as to any of the requirements of this Agreement
under the circumstance before it, delay or refrain from taking any action unless
and until it shall have received appropriate written instructions from the
Trustee or advice from legal counsel selected by it (or other appropriate
advisor), as the case may be, detailing the action required to be taken
hereunder and the Account Holder may rely conclusively on any such instructions
or advice;

                  (iii) so long as it and they shall have acted (or refrained
from acting) in good faith and within the reasonable belief that such action or
omission is duly authorized or within the discretion or powers granted to it
hereunder, shall not be responsible or liable for any error of judgment in any
action taken, suffered or omitted by it or them, or for any act done or step
taken or omitted, or for any mistake of fact or law, unless such action
constitutes gross negligence or willful misconduct as finally determined by a
non-appealable judgment of a court of competent jurisdiction on its (or their)
part;

                  (iv)  will not be responsible or liable to the Pledgor, the
Trustee, or any other person or entity whatsoever for the due execution,
legality, validity, enforceability, genuineness, effectiveness or sufficiency of
this Agreement (provided, however, that the Account Holder warrants that the
Account Holder has legal capacity and has been duly authorized to enter into
this Agreement) or for any statement, warranty or representation made by any
other party in connection with this Agreement;

                  (v)   will not incur any responsibility or liability by acting
or not acting in reliance upon advice of counsel, or upon any notice, consent,
certificate, instruction, Account Direction, statement, wire instruction,
telecopy or other writing reasonably and in good faith believed by it or them to
be genuine and in conformance with this Agreement and signed or sent by the
proper party or parties and contemplated herein; and

                  (vi)  shall not be required to expend or risk its or their own
funds, or to take any action (including the institution or defense of legal
proceedings) which in its or their reasonable judgment may cause it or them to
incur or suffer any expense or liability, unless the

                                      A - 4
<PAGE>

Account Holder shall have been provided with security or indemnity, acceptable
to Account Holder in its sole discretion, for the payment of the costs, expenses
(including reasonable attorneys' fees) and liabilities which may be incurred
therein or thereby.

            (e) If any Collateral subject to this Agreement is at any time
attached or levied upon, or in case the transfer or delivery of any such
Collateral shall be stayed or enjoined, or in the case of any other legal
process or judicial order affecting such Collateral, the Account Holder is
authorized to comply with any such order in any manner as the Account Holder or
its legal counsel reasonably deems appropriate. The Account Holder shall give
prompt written notice to the Pledgor and the Trustee of any such attachment,
levy, stay, injunction or legal process. If the Account Holder complies with any
process, order, writ, judgment or decree relating to the Collateral subject to
this Agreement, then the Account Holder shall not be liable or responsible to
the Pledgor, the Trustee, or any other person or entity whatsoever even if such
order, writ, judgment, decree or process is subsequently modified, vacated or
otherwise determined to have been without legal force or effect.

            (f) The Account Holder shall not be liable or responsible for any
delays or failures in performance of any of its duties hereunder which result
from events or conditions beyond its reasonable control and so long as the same
exist or continue and cannot reasonably be remedied by the Account Holder in
accordance with its normal business practices. Such events or conditions shall
include, but shall not be limited to, acts of God, strikes, lockouts, riots,
acts of war or terrorism, epidemics, nationalization, expropriation, currency
restrictions, governmental regulations superimposed after the fact, fire,
communication line failures (including the unavailability of the Federal Reserve
Bank wire or telex or other wire or communication facility), power failures,
earthquakes or other disasters.

            (g) Subject to the provisions hereof, so long as the Account Holder
and the Trustee are the same institution, the Account Holder may rely on written
directions given by the Pledgor to the Trustee (including but not limited to
Issuer Orders) and shall not require separate corresponding instructions from
the Trustee (including but not limited to Payment Orders).

            SECTION 7. Indemnity. The Pledgor will indemnify the Account Holder,
its officers, directors, employees and agents against claims, liabilities and
expenses arising out of this Agreement (including, without limitation,
reasonable attorney's fees and disbursements), except to the extent the claims,
liabilities or expenses are caused by the Account Holder's gross negligence or
willful misconduct.

            SECTION 8. Termination; Survival. (a) This Agreement shall terminate
automatically upon receipt by the Account Holder of written notice executed by
an authorized officer of the Trustee that (i) all of the Secured Obligations
have been paid in full in cash or otherwise satisfied or (ii) all of the
Collateral has been released, whichever is earlier, and the Account Holder shall
thereafter be relieved of all duties and obligations hereunder. The Account
Holder may terminate this Agreement on 60 days' prior notice to the Trustee and
the Pledgor, provided that before such termination the Account Holder and the
Pledgor shall make arrangements to transfer the property in the Pledged Account
to another securities intermediary that shall have executed, together with the
Trustee and the Pledgor, a control agreement in favor

                                      A - 5
<PAGE>

of the Trustee and the Holders of the Notes in respect of such property in
substantially the form of this Agreement or otherwise in form and substance
satisfactory to the Trustee.

            (b) In the event that the Trustee no longer serves as Trustee for
the Collateral, the Trustee, the Account Holder and the Pledgor shall make
arrangements for another Person to assume the rights and obligations of the
Trustee hereunder, and such Person shall have executed, together with the
Account Holder and the Pledgor, a control agreement in favor of such Person and
the Holders of the Notes in substantially the form of this Agreement or
otherwise in form and substance satisfactory to the Trustee.

            (c) Sections 7 and 8 will survive termination of this Agreement.

            SECTION 9. Conflict with Other Agreements. (a) In the event of any
conflict between this Agreement (or any portion thereof) and any other agreement
now existing or hereafter entered into, the terms of this Agreement shall
prevail.

            (b) The Account Holder hereby confirms and agrees that:

                  (i)   There are no other agreements entered into between the
Account Holder and the Pledgor with respect to the Pledged Account;

                  (ii)  It has not entered into, and until the termination of
this Agreement will not enter into, any agreement with any other person relating
to the Pledged Account and/or any financial assets credited thereto or funds
therein pursuant to which it has agreed to comply with entitlement orders (as
defined in Section 8-102(a)(8) of the UCC) of such other person; and

                  (iii) It has not entered into, and until the termination of
this Agreement will not enter into, any agreement with the Pledgor or the
Trustee purporting to limit or condition the obligation of the Account Holder to
comply with Account Directions as set forth in Section 3 hereof.

            SECTION 10. Permitted Investments. In accordance with the Pledge
Agreement, the Trustee shall direct the Account Holder with respect to the
selection of investments to be made with the funds in the Pledged Account.

            SECTION 11. Entire Agreement. This Agreement is the entire
agreement, and supersedes any prior agreements, and contemporaneous oral
agreements, of the parties concerning its subject matter. The Trustee and the
Account Holder shall be entitled to all the rights, benefits, privileges and
immunities accorded to the Trustee under the Indenture.

            SECTION 12. Amendments. No modification, amendment or waiver of, nor
consent to any departure by any party from, any provision of this Agreement will
be effective unless made in writing signed by the parties hereto, and then such
waiver or consent shall be effective only in the specific instance and for the
purpose for which given.

                                      A - 6
<PAGE>

            SECTION 13. Financial Assets. The Account Holder agrees with Trustee
and the Pledgor that all property credited from time to time to the Pledged
Account will be treated as financial assets under Article 8 of the UCC.

            SECTION 14. Notices. All notices, demands, requests, consents,
approvals and other communications required or permitted hereunder must be in
writing and will be effective upon receipt if delivered personally, or if sent
by facsimile transmission with confirmation of delivery, or by nationally
recognized overnight courier service, to the Pledgor's and the Trustee's
addresses as set forth in the Pledge Agreement, and to the Account Holder's
address as set forth on the signature page below, or to such other address as
any party may give to the others in writing for such purpose.

            SECTION 15. Binding Effect. This Agreement shall become effective
when it shall have been executed by the Pledgor, the Trustee and the Account
Holder, and thereafter shall be binding upon and inure to the benefit of the
Pledgor, the Trustee and the Account Holder and their respective successors and
assigns.

            SECTION 16. Execution in Counterparts. This Agreement may be
executed in any number of counterparts and by different parties hereto in
separate counterparts, each of which when so executed shall be deemed to be an
original and all of which taken together shall constitute one and the same
agreement. Delivery of an executed counterpart of a signature page to this
Agreement by telecopier shall be effective as delivery of an original executed
counterpart of this Agreement.

            SECTION 17. Governing Law and Jurisdiction. THIS AGREEMENT WILL BE
GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW
YORK. Each of the parties hereby irrevocably submits for itself and its property
in any legal action or proceeding relating to this Agreement, or for recognition
and enforcement of any judgment in respect thereof, to the non-exclusive general
jurisdiction and venue of the courts of the State of New York, the courts of the
United States of America in New York, and appellate courts from any thereof.

            SECTION 18. WAIVER OF JURY TRIAL. EACH OF THE PARTIES HERETO
IRREVOCABLY WAIVES ANY AND ALL RIGHT IT MAY HAVE TO A TRIAL BY JURY IN ANY
ACTION, PROCEEDING OR CLAIM (WHETHER BASED ON CONTRACT, TORT OR OTHERWISE) OF
ANY NATURE RELATING TO THIS AGREEMENT, ANY DOCUMENTS EXECUTED IN CONNECTION WITH
THIS AGREEMENT OR ANY TRANSACTION CONTEMPLATED IN ANY OF SUCH DOCUMENTS. EACH
PARTY HERETO ACKNOWLEDGES THAT THE FOREGOING WAIVER IS KNOWING AND VOLUNTARY.

                                      A - 7
<PAGE>

            IN WITNESS WHEREOF, the parties hereto have caused this Agreement to
be executed by their respective officers thereunto duly authorized, as of the
date first above written.

                                Pledgor:

                                THORATEC CORPORATION

                                   By: /s/ D. Keith Grossman
                                       ----------------------------------------
                                   Name: D. Keith Grossman
                                   Title: President and Chief Executive Officer

                    [Signature page to the Control Agreement]

                                      A - 8
<PAGE>

            IN WITNESS WHEREOF, the parties hereto have caused this Agreement to
be executed by their respective officers thereunto duly authorized, as of the
date first above written.

                                Trustee:

                                U.S. BANK NATIONAL ASSOCIATION

                                By: /s/ Paula M. Oswald
                                    --------------------------------------------
                                    Name: Paula M. Oswald
                                    Title: Vice President

                    [Signature page to the Control Agreement]

                                      A - 9
<PAGE>

            A - 10 IN WITNESS WHEREOF, the parties hereto have caused this
Agreement to be executed by their respective officers thereunto duly authorized,
as of the date first above written.

                                Account Holder:

                                U.S. BANK NATIONAL ASSOCIATION

                                By: /s/ Paula M. Oswald
                                    --------------------------------------------
                                    Name: Paula M. Oswald
                                    Title: Vice President

                                Address for Notices:

                                U.S. Bank National Association
                                633 West Fifth Street, 24th Floor
                                Los Angeles, California 90071
                                Fax:  (213) 615-6197
                                Attention: Paula Oswald

                    [Signature page to the Control Agreement]

                                     A - 10
<PAGE>

                                   SCHEDULE I

                            Pledged Financial Assets
<TABLE>
<CAPTION>
Par Amount                Maturity Date                 CUSIP No.
----------                -------------                 ---------
<S>                     <C>                             <C>
1,419,000               November 15, 2004               912833FV7

1,485,000                  May 15, 2005                 912833FW5

1,485,000               November 15, 2005               912833FX3

1,485,000                  May 15, 2006                 912833FY1

1,485,000               November 15, 2006               912833FZ8

1,485,000                  May 15, 2007                 912833GA2
</TABLE>

                                     A - 11
<PAGE>

                           PLEDGE AGREEMENT SUPPLEMENT

            This PLEDGE AGREEMENT SUPPLEMENT (this "Supplement") is made and
entered into as of June 7, 2004 by Thoratec Corporation, a California
corporation (the "Pledgor"), having its principal office at 6035 Stoneridge
Drive, Pleasanton, California 94588, in favor of U.S. Bank National Association,
in its capacity as trustee (the "Trustee") for the holders from time to time
(the "Holders") of the Notes (as defined in the Pledge Agreement referred to
below), issued by the Pledgor under the Indenture referred to below.

                             PRELIMINARY STATEMENTS:

            (1)   The Pledgor and the Trustee have entered into the Pledge
Agreement dated as of May 24, 2004 (as amended, restated, supplemented or
otherwise modified from time to time, the "Pledge Agreement"). Capitalized terms
used and not defined in this Supplement have the meanings set forth or referred
to in the Pledge Agreement.

            (2)   The Pledgor and Merrill Lynch & Co., Merrill Lynch, Pierce,
Fenner & Smith Incorporated as the initial purchaser (the "Initial Purchaser")
are party to the Purchase Agreement dated May 18, 2004 (the "Purchase
Agreement"), pursuant to which the Pledgor has issued and sold to the Initial
Purchaser $215,154,000 aggregate principal amount at maturity of Senior
Subordinated Convertible Notes due 2034 (the "Initial Notes") and pursuant to
which the Pledgor has granted to the Initial Purchaser an option to purchase all
or any part of an additional $32,273,000 aggregate principal amount at maturity
of Senior Subordinated Convertible Notes due 2034 (the "Option Notes" and,
together with the Initial Notes, the "Notes").

            (3)   The Pledgor and the Trustee have entered into the Indenture
dated as of May 24, 2004 (as amended, restated, supplemented or otherwise
modified from time to time, the "Indenture"), pursuant to which the Pledgor
issued the Initial Notes on May 24, 2004 and pursuant to which the Pledgor is
issuing the Option Notes on the date hereof.

            (4)   The Initial Purchaser exercised its option to purchase all of
the Option Notes, and the Pledgor has issued and sold to the Initial Purchaser
the Option Notes on the date hereof.

            (5)   The Pledgor is the beneficial owner of, and desires to pledge
additional security entitlements (the "Additional Pledged Security
Entitlements") with respect to the United States Treasury securities identified
by CUSIP number as set forth on Exhibit A hereto, and credited to the Pledge
Account; and

            (6)   To secure all Obligations in respect of the Option Notes, the
Pledgor has agreed (i) to pledge to the Trustee for its benefit and the ratable
benefit of the Holders of the Option Notes, a security interest in additional
Collateral, including the Additional Pledged Security Entitlements, securing the
payment and performance by the Pledgor of all of the Obligations in respect of
the Option Notes and (ii) to execute and deliver this Supplement.

            NOW THEREFORE, the parties hereto agree as follows:
<PAGE>

            SECTION 1. The Pledgor hereby acknowledges and confirms that (a) the
Additional Pledged Security Entitlements have been duly pledged as Pledged
Financial Assets under the Pledge Agreement and are subject to the security
interest granted under the Pledge Agreement and (b) the Additional Pledged
Security Entitlements are subject to the terms and provisions of the Pledge
Agreement as Pledged Financial Assets thereunder.

            SECTION 2. Schedule I to the Pledge Agreement is hereby amended by
deleting such Schedule in its entirety and replacing it with Schedule I as
attached hereto.

            SECTION 3. On and after the date of this Supplement (i) each
reference in the Pledge Agreement to "this Pledge Agreement", "hereunder",
"hereof" or words of like import referring to the Pledge Agreement shall mean
and be a reference to the Pledge Agreement as supplemented by this Supplement
and (ii) each reference in the Pledge Agreement to "Schedule I" shall mean and
be a reference to Schedule I as amended by this Supplement.

            SECTION 4. Except for the modifications explicitly set forth in this
Supplement, no other amendments or modifications have been made to the Pledge
Agreement hereby and all other terms and provisions of the Pledge Agreement
shall remain in full force and effect.

            SECTION 5. This Supplement may be executed in any number of
counterparts and by different parties hereto in separate counterparts, each of
which when so executed shall be deemed to be an original and all of which taken
together shall constitute one and the same agreement. Delivery of an executed
counterpart of a signature page to this Supplement by telecopier shall be
effective as delivery of an original executed counterpart of this Supplement.

            SECTION 6. This Supplement shall be governed by, and construed in
accordance with, the laws of the State of New York.

<PAGE>

            IN WITNESS WHEREOF, the Pledgor and the Trustee have each caused
this Pledge Agreement Supplement to be duly executed and delivered as of the
date first above written.

                                Pledgor:

                                THORATEC CORPORATION

                                By: /s/ D. Keith Grossman
                                    --------------------------------------------
                                Name: D. Keith Grossman

                                Title: President and Chief Executive Officer

                                Trustee:

                                U.S. BANK NATIONAL ASSOCIATION

                                By: /s/ Paula M. Oswald
                                    --------------------------------------------
                                Name: Paula M. Oswald
                                Title: Vice President

               [Signature page to the Pledge Agreement Supplement]

<PAGE>

                                                                       EXHIBIT A

                    Additional Pledged Security Entitlements

<TABLE>
<CAPTION>
Par Amount                       Maturity Date             CUSIP No.
-----------                    -----------------           ---------
<S>                            <C>                         <C>
  213,000                      November 15, 2004           912833FV7

  223,000                         May 15, 2005             912833FW5

  223,000                      November 15, 2005           912833FX3

  223,000                         May 15, 2006             912833FY1

  223,000                      November 15, 2006           912833FZ8

  223,000                         May 15, 2007             912833GA2
</TABLE>

                                   Exhibit A

<PAGE>

                                   SCHEDULE I

                            Pledged Financial Assets

                      Initial Pledged Security Entitlements

<TABLE>
<CAPTION>
Par Amount                       Maturity Date             CUSIP No.
-----------                    -----------------           ---------
<S>                            <C>                         <C>
1,419,000                      November 15, 2004           912833FV7

1,485,000                         May 15, 2005             912833FW5

1,485,000                      November 15, 2005           912833FX3

1,485,000                         May 15, 2006             912833FY1

1,485,000                      November 15, 2006           912833FZ8

1,485,000                         May 15, 2007             912833GA2
</TABLE>

                     Additional Pledged Security Entitlement

<TABLE>
<S>                            <C>                         <C>
 213,000                       November 15, 2004           912833FV7

 223,000                          May 15, 2005             912833FW5

 223,000                       November 15, 2005           912833FX3

 223,000                          May 15, 2006             912833FY1

 223,000                       November 15, 2006           912833FZ8

 223,000                          May 15, 2007             912833GA2
</TABLE>

                                   Schdeule I<PAGE>

                                                                     EXHIBIT 4.3

                                                                  EXECUTION COPY

                                CONTROL AGREEMENT

            This CONTROL AGREEMENT (this "Agreement") dated as of May 24, 2004
by and among Thoratec Corporation (the "Pledgor") and U.S. Bank National
Association, in its capacity as trustee (the "Trustee"), and U.S. Bank National
Association, a national banking association, in its capacity as securities
intermediary and depository bank (the "Account Holder").

                             PRELIMINARY STATEMENTS:

            (1)   The Pledgor has granted the Trustee a security interest (the
"Security Interest") in certain security entitlements (the "Pledged Security
Entitlements") with respect to certain U.S. Treasury securities (the "Pledged
Financial Assets") identified on Schedule I attached hereto maintained by the
Trustee with the Account Holder and carried from time to time in an account with
the Account Holder, ABA No. 091000022, Account No. 785834000, and entitled
"Thoratec Corporation Senior Subordinated Convertible Notes due 2034, Collateral
Pledge Account" (the "Pledged Account") and all additions thereto and
substitutions and proceeds thereof (collectively, the "Collateral"), pursuant
to, and as more particularly described in, a Pledge Agreement dated as of May
18, 2004, among the Pledgor and the Trustee (as the same may hereafter be
amended, supplemented or otherwise modified from time to time, the "Pledge
Agreement"; terms defined in the Pledge Agreement and not otherwise defined
herein are used herein as therein defined). The Pledgor acknowledges having
received value for such pledge of the Collateral.

            (2)   Terms defined in Article 8 or 9 of the Uniform Commercial Code
as in effect in the State of New York (the "UCC") are used in this Agreement
(including, without limitation, paragraph (1) above) as such terms are defined
in such Article 8 or 9.

            (3)   The Pledgor, the Trustee and the Account Holder are delivering
this Agreement pursuant to the terms of the Pledge Agreement.

            NOW, THEREFORE, in consideration of the premises and mutual
agreements contained herein, the parties hereto hereby agree as follows:

            SECTION 1. Notice of Exclusive Control. The Pledgor and Trustee are
entering into this Agreement to perfect, and confirm the first priority lien of,
the Trustee's Security Interest in the Collateral. The Account Holder agrees to
promptly make all necessary entries or notations in its books and records to
reflect the Trustee's Security Interest in the Collateral and to apply any value
distributed on account of any Pledged Financial Assets as directed in writing by
the Trustee without further consent from the Pledgor. The Account Holder
acknowledges that the Trustee has exclusive control over the Pledged Account and
all Pledged Security Entitlements contained therein from time to time.

            SECTION 2. The Account. The Account Holder represents and warrants
to, and agrees with, the Pledgor and the Trustee and the Holders of the Notes
that:

                                       1
<PAGE>

            (a)   The Account Holder has established the Pledged Account and
shall not change the name or account number of the Pledged Account without the
prior written consent of the Trustee.

            (b)   The Account Holder maintains the Pledged Account for the
Trustee, and all property (including, without limitation, all funds and
financial assets) held by the Account Holder for the account of the Trustee is,
and will continue to be, credited to the Pledged Account. The Account Holder is
a securities intermediary (as defined in Section 8-102(a)(xiv) of the UCC) and
is acting in such capacity in relation to the Trustee, the Pledgor and the
Pledged Account and with respect to all property credited thereto, from time to
time.

            (c)   The Pledged Account is a securities account. The Account
Holder is the securities intermediary with respect to financial assets credited
to the Pledged Account from time to time. The Trustee is the entitlement holder
with respect to financial assets credited from time to time to the Pledged
Account.

            (d)   All financial assets in registered form or payable to or to
the order of and credited to the Pledged Account shall be registered in the name
of, payable to or to the order of, or endorsed to, the Account Holder and in no
case during the term of the Pledge Agreement will any financial asset credited
to the Pledged Account be registered in the name of, payable to or to the order
of, or endorsed to, the Pledgor, except to the extent the foregoing have been
subsequently endorsed by the Pledgor to the Account Holder or in blank.

            (e)   Notwithstanding any other agreement to the contrary, the
Account Holder's jurisdiction with respect to the Pledged Account for purposes
of the UCC is, and will continue to be for so long as the Security Interest
shall be in effect, the State of New York.

            (f)   The Account Holder does not know of any claim to or interest
in the Pledged Account or any property (including, without limitation, all funds
and financial assets) credited to the Pledged Account, except for claims and
interests of the parties referred to in this Agreement.

            SECTION 3. Control by Trustee. (a) The Account Holder will comply
with (A) all written instructions directing disposition of the funds in the
Pledged Account (such instructions, a "Payment Order"), (B) all notifications
and entitlement orders that the Account Holder receives directing it to transfer
or redeem any financial asset in the Pledged Account and (C) all other
directions concerning the Collateral, including, without limitation, directions
to distribute to the Trustee proceeds of any such transfer or redemption or
interest on any property in the Pledged Account (any such instruction,
notification or direction referred to in clause (A), (B) or (C) above being an
"Account Direction"), in each case of clauses (A), (B) and (C) above originated
by the Trustee without further consent by the Pledgor or any other person.

            (b)   The Trustee hereby acknowledges that it shall maintain and
exercise control of the Pledged Account on behalf of the Holders of the Notes.

            (c)   The Account Holder will not (i) comply with Account Directions
or other directions concerning the Collateral originated by the Pledgor or (ii)
distribute to the Pledgor

                                       2
<PAGE>

interest or other distributions on or in respect of the Collateral, unless
instructed to do so by the Trustee, in its absolute discretion.

            SECTION 4. Priority of Trustee's Security Interest. (a) The Account
Holder (i) hereby subordinates to the Security Interest and in favor of the
Trustee any security interest, lien, or right of setoff the Account Holder may
have, now or in the future, against the Pledged Account or property in the
Pledged Account, and (ii) agrees that it will not exercise any right in respect
of any such security interest or lien or any such right of setoff until the
Security Interest is terminated, except that the Account Holder will retain its
prior lien on property in the Pledged Account to secure payment for property
purchased for the Pledged Account and normal commissions and fees for the
Pledged Account.

            (b)   The Account Holder will not enter into any other agreement
with any Person relating to Account Directions or other directions with respect
to the Pledged Account.

            SECTION 5. Statements, Confirmations, and Notices of Adverse Claims.
(a) The Account Holder will send copies of all statements and confirmations for
the Pledged Account simultaneously to the Pledgor and the Trustee. So long as
the Trustee and the Account Holder are the same institution, the Account Holder
is not required to send copies of statements and confirmations as provided in
the preceding sentence.

            (b)   When the Account Holder knows of any claim or interest in the
Pledged Account or any property credited to the Pledged Account other than the
claims and interests of the parties referred to in this Agreement, the Account
Holder will promptly notify the Trustee and the Pledgor of such claim or
interest.

            SECTION 6. The Account Holder's Responsibility. (a) The Account
Holder will not be liable to the Pledgor or the Trustee or the Holders of the
Notes for complying with an Account Direction or other direction concerning the
Collateral originated by the Trustee, even if the Pledgor notifies the Account
Holder that the Trustee is not legally entitled to issue the Account Direction
or such other direction unless the Account Holder takes the action after it is
served with an injunction, restraining order, or other legal process enjoining
it from doing so, issued by a court of competent jurisdiction, and had a
reasonable opportunity to act on the injunction, restraining order or other
legal process.

            (b)   This Agreement does not create any obligation of the Account
Holder except for those expressly set forth in this Agreement, in Part 5 of
Article 8 of the UCC, or in Article 4 of the UCC. In particular, the Account
Holder need not investigate whether the Trustee is entitled under the Trustee's
agreements with the Pledgor to give an Account Direction or other direction
concerning the Pledged Account. The Account Holder may conclusively rely on
notices and communications it believes given by the appropriate party.

            (c)   In no event shall the Account Holder or any of its affiliates,
shareholders, directors, officers, employees or agents be liable for indirect,
special, punitive, incidental or consequential damages of any kind whatsoever
even if advised of the possibility of such damages, other than such damages
caused by its own bad faith, gross negligence or willful misconduct.

                                       3
<PAGE>

            (d)   Without limiting the foregoing, and notwithstanding any
provision to the contrary elsewhere, the Account Holder and its affiliates,
shareholders, directors, officers, employees or agents:

                  (i)   shall have no responsibilities, obligations or duties in
respect of the subject matter hereof other than those expressly set forth in
this Agreement, and no implied duties, responsibilities, covenants or
obligations shall be read into this Agreement against the Account Holder.
Without limiting the foregoing, the Account Holder shall have no duty or
authority to determine and/or investigate whether or not an event of default
exists under any agreement between the Pledgor and the Trustee, or to determine
and/or investigate whether or not the Trustee is entitled to give any Account
Direction with respect to the Collateral;

                  (ii)  may in any instance where the Account Holder determines
that it lacks or is uncertain as to its authority to take or refrain from taking
certain action hereunder, or as to any of the requirements of this Agreement
under the circumstance before it, delay or refrain from taking any action unless
and until it shall have received appropriate written instructions from the
Trustee or advice from legal counsel selected by it (or other appropriate
advisor), as the case may be, detailing the action required to be taken
hereunder and the Account Holder may rely conclusively on any such instructions
or advice;

                  (iii) so long as it and they shall have acted (or refrained
from acting) in good faith and within the reasonable belief that such action or
omission is duly authorized or within the discretion or powers granted to it
hereunder, shall not be responsible or liable for any error of judgment in any
action taken, suffered or omitted by it or them, or for any act done or step
taken or omitted, or for any mistake of fact or law, unless such action
constitutes gross negligence or willful misconduct as finally determined by a
non-appealable judgment of a court of competent jurisdiction on its (or their)
part;

                  (iv)  will not be responsible or liable to the Pledgor, the
Trustee, or any other person or entity whatsoever for the due execution,
legality, validity, enforceability, genuineness, effectiveness or sufficiency of
this Agreement (provided, however, that the Account Holder warrants that the
Account Holder has legal capacity and has been duly authorized to enter into
this Agreement) or for any statement, warranty or representation made by any
other party in connection with this Agreement;

                  (v)   will not incur any responsibility or liability by acting
or not acting in reliance upon advice of counsel, or upon any notice, consent,
certificate, instruction, Account Direction, statement, wire instruction,
telecopy or other writing reasonably and in good faith believed by it or them to
be genuine and in conformance with this Agreement and signed or sent by the
proper party or parties and contemplated herein; and

                  (vi)  shall not be required to expend or risk its or their own
funds, or to take any action (including the institution or defense of legal
proceedings) which in its or their reasonable judgment may cause it or them to
incur or suffer any expense or liability, unless the Account Holder shall have
been provided with security or indemnity, acceptable to Account Holder in its
sole discretion, for the payment of the costs, expenses (including reasonable
attorneys' fees) and liabilities which may be incurred therein or thereby.

                                       4
<PAGE>

            (e)   If any Collateral subject to this Agreement is at any time
attached or levied upon, or in case the transfer or delivery of any such
Collateral shall be stayed or enjoined, or in the case of any other legal
process or judicial order affecting such Collateral, the Account Holder is
authorized to comply with any such order in any manner as the Account Holder or
its legal counsel reasonably deems appropriate. The Account Holder shall give
prompt written notice to the Pledgor and the Trustee of any such attachment,
levy, stay, injunction or legal process. If the Account Holder complies with any
process, order, writ, judgment or decree relating to the Collateral subject to
this Agreement, then the Account Holder shall not be liable or responsible to
the Pledgor, the Trustee, or any other person or entity whatsoever even if such
order, writ, judgment, decree or process is subsequently modified, vacated or
otherwise determined to have been without legal force or effect.

            (f)   The Account Holder shall not be liable or responsible for any
delays or failures in performance of any of its duties hereunder which result
from events or conditions beyond its reasonable control and so long as the same
exist or continue and cannot reasonably be remedied by the Account Holder in
accordance with its normal business practices. Such events or conditions shall
include, but shall not be limited to, acts of God, strikes, lockouts, riots,
acts of war or terrorism, epidemics, nationalization, expropriation, currency
restrictions, governmental regulations superimposed after the fact, fire,
communication line failures (including the unavailability of the Federal Reserve
Bank wire or telex or other wire or communication facility), power failures,
earthquakes or other disasters.

            (g)   Subject to the provisions hereof, so long as the Account
Holder and the Trustee are the same institution, the Account Holder may rely on
written directions given by the Pledgor to the Trustee (including but not
limited to Issuer Orders) and shall not require separate corresponding
instructions from the Trustee (including but not limited to Payment Orders).

            SECTION 7. Indemnity. The Pledgor will indemnify the Account Holder,
its officers, directors, employees and agents against claims, liabilities and
expenses arising out of this Agreement (including, without limitation,
reasonable attorney's fees and disbursements), except to the extent the claims,
liabilities or expenses are caused by the Account Holder's gross negligence or
willful misconduct.

            SECTION 8. Termination; Survival. (a) This Agreement shall terminate
automatically upon receipt by the Account Holder of written notice executed by
an authorized officer of the Trustee that (i) all of the Secured Obligations
have been paid in full in cash or otherwise satisfied or (ii) all of the
Collateral has been released, whichever is earlier, and the Account Holder shall
thereafter be relieved of all duties and obligations hereunder. The Account
Holder may terminate this Agreement on 60 days' prior notice to the Trustee and
the Pledgor, provided that before such termination the Account Holder and the
Pledgor shall make arrangements to transfer the property in the Pledged Account
to another securities intermediary that shall have executed, together with the
Trustee and the Pledgor, a control agreement in favor of the Trustee and the
Holders of the Notes in respect of such property in substantially the form of
this Agreement or otherwise in form and substance satisfactory to the Trustee.

                                       5
<PAGE>

            (b)   In the event that the Trustee no longer serves as Trustee for
the Collateral, the Trustee, the Account Holder and the Pledgor shall make
arrangements for another Person to assume the rights and obligations of the
Trustee hereunder, and such Person shall have executed, together with the
Account Holder and the Pledgor, a control agreement in favor of such Person and
the Holders of the Notes in substantially the form of this Agreement or
otherwise in form and substance satisfactory to the Trustee.

            (c)   Sections 7 and 8 will survive termination of this Agreement.

            SECTION 9. Conflict with Other Agreements. (a) In the event of any
conflict between this Agreement (or any portion thereof) and any other agreement
now existing or hereafter entered into, the terms of this Agreement shall
prevail.

            (b)   The Account Holder hereby confirms and agrees that:

                  (i)   There are no other agreements entered into between the
Account Holder and the Pledgor with respect to the Pledged Account;

                  (ii)  It has not entered into, and until the termination of
this Agreement will not enter into, any agreement with any other person relating
to the Pledged Account and/or any financial assets credited thereto or funds
therein pursuant to which it has agreed to comply with entitlement orders (as
defined in Section 8-102(a)(8) of the UCC) of such other person; and

                  (iii) It has not entered into, and until the termination of
this Agreement will not enter into, any agreement with the Pledgor or the
Trustee purporting to limit or condition the obligation of the Account Holder to
comply with Account Directions as set forth in Section 3 hereof.

            SECTION 10. Permitted Investments. In accordance with the Pledge
Agreement, the Trustee shall direct the Account Holder with respect to the
selection of investments to be made with the funds in the Pledged Account.

            SECTION 11. Entire Agreement. This Agreement is the entire
agreement, and supersedes any prior agreements, and contemporaneous oral
agreements, of the parties concerning its subject matter. The Trustee and the
Account Holder shall be entitled to all the rights, benefits, privileges and
immunities accorded to the Trustee under the Indenture.

            SECTION 12. Amendments. No modification, amendment or waiver of, nor
consent to any departure by any party from, any provision of this Agreement will
be effective unless made in writing signed by the parties hereto, and then such
waiver or consent shall be effective only in the specific instance and for the
purpose for which given.

            SECTION 13. Financial Assets. The Account Holder agrees with Trustee
and the Pledgor that all property credited from time to time to the Pledged
Account will be treated as financial assets under Article 8 of the UCC.

                                       6
<PAGE>

            SECTION 14. Notices. All notices, demands, requests, consents,
approvals and other communications required or permitted hereunder must be in
writing and will be effective upon receipt if delivered personally, or if sent
by facsimile transmission with confirmation of delivery, or by nationally
recognized overnight courier service, to the Pledgor's and the Trustee's
addresses as set forth in the Pledge Agreement, and to the Account Holder's
address as set forth on the signature page below, or to such other address as
any party may give to the others in writing for such purpose.

            SECTION 15. Binding Effect. This Agreement shall become effective
when it shall have been executed by the Pledgor, the Trustee and the Account
Holder, and thereafter shall be binding upon and inure to the benefit of the
Pledgor, the Trustee and the Account Holder and their respective successors and
assigns.

            SECTION 16. Execution in Counterparts. This Agreement may be
executed in any number of counterparts and by different parties hereto in
separate counterparts, each of which when so executed shall be deemed to be an
original and all of which taken together shall constitute one and the same
agreement. Delivery of an executed counterpart of a signature page to this
Agreement by telecopier shall be effective as delivery of an original executed
counterpart of this Agreement.

            SECTION 17. Governing Law and Jurisdiction. THIS AGREEMENT WILL BE
GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW
YORK. Each of the parties hereby irrevocably submits for itself and its property
in any legal action or proceeding relating to this Agreement, or for recognition
and enforcement of any judgment in respect thereof, to the non-exclusive general
jurisdiction and venue of the courts of the State of New York, the courts of the
United States of America in New York, and appellate courts from any thereof.

            SECTION 18. WAIVER OF JURY TRIAL. EACH OF THE PARTIES HERETO
IRREVOCABLY WAIVES ANY AND ALL RIGHT IT MAY HAVE TO A TRIAL BY JURY IN ANY
ACTION, PROCEEDING OR CLAIM (WHETHER BASED ON CONTRACT, TORT OR OTHERWISE) OF
ANY NATURE RELATING TO THIS AGREEMENT, ANY DOCUMENTS EXECUTED IN CONNECTION WITH
THIS AGREEMENT OR ANY TRANSACTION CONTEMPLATED IN ANY OF SUCH DOCUMENTS. EACH
PARTY HERETO ACKNOWLEDGES THAT THE FOREGOING WAIVER IS KNOWING AND VOLUNTARY.

                                       7
<PAGE>

            IN WITNESS WHEREOF, the parties hereto have caused this Agreement to
be executed by their respective officers thereunto duly authorized, as of the
date first above written.

                                Pledgor:

                                THORATEC CORPORATION

                                By: /s/ D. Keith Grossman
                                    --------------------------------------------
                                Name: D. Keith Grossman
                                Title: President and Chief Executive Officer

                    [Signature page to the Control Agreement]

<PAGE>

            IN WITNESS WHEREOF, the parties hereto have caused this Agreement to
be executed by their respective officers thereunto duly authorized, as of the
date first above written.

                                Trustee:

                                U.S. BANK NATIONAL ASSOCIATION

                                By: /s/ Paula M. Oswald
                                    --------------------------------------------
                                    Name: Paula M. Oswald
                                    Title: Vice President

                   [Signature page to the Control Agreement]

<PAGE>

            IN WITNESS WHEREOF, the parties hereto have caused this Agreement to
be executed by their respective officers thereunto duly authorized, as of the
date first above written.

                                Account Holder:

                                U.S. BANK NATIONAL ASSOCIATION

                                By: /s/ Paula M. Oswald
                                    --------------------------------------------
                                    Name: Paula M. Oswald
                                    Title: Vice President

                                Address for Notices:

                                U.S. Bank National Association
                                633 West Fifth Street, 24th Floor
                                Los Angeles, California 90071
                                Fax: (213) 615-6197
                                Attention: Paula Oswald

                   [Signature page to the Control Agreement]

<PAGE>

                                   SCHEDULE I

                            Pledged Financial Assets
<TABLE>
<CAPTION>
Par Amount                 Maturity Date              CUSIP No.
----------                 -------------              ---------
<S>                      <C>                          <C>
1,419,000                November 15, 2004            912833FV7

1,485,000                   May 15, 2005              912833FW5

1,485,000                November 15, 2005            912833FX3

1,485,000                   May 15, 2006              912833FY1

1,485,000                November 15, 2006            912833FZ8

1,485,000                   May 15, 2007              912833GA2
</TABLE>

                                    Sch - 1
<PAGE>

                           CONTROL AGREEMENT AMENDMENT

            This CONTROL AGREEMENT AMENDMENT (this "Amendment") dated as of June
7, 2004 by and among Thoratec Corporation (the "Pledgor") and U.S. Bank National
Association, in its capacity as trustee (the "Trustee"), and U.S. Bank National
Association, a national banking association, in its capacity as securities
intermediary and depository bank (the "Account Holder").

                             PRELIMINARY STATEMENTS:

            (1)   The Pledgor, the Trustee and the Account Holder have entered
into the Control Agreement dated as of May 24, 2004 (as amended, restated,
supplemented or otherwise modified from time to time, the "Control Agreement").
Capitalized terms used and not defined in this Amendment have the meanings set
forth or referred to in the Control Agreement.

            (2)   The Pledgor is the beneficial owner of, and desires to pledge
additional security entitlements (the "Additional Pledged Security
Entitlements") with respect to the United States Treasury securities identified
by CUSIP number as set forth on Exhibit A hereto, and credited to the Pledge
Account; and

            (3)   To secure all Obligations in respect of the Option Notes, the
Pledgor has agreed to pledge to the Trustee for its benefit and the ratable
benefit of the Holders of the Option Notes, a security interest in additional
Collateral, including the Additional Pledged Security Entitlements, securing the
payment and performance by the Pledgor of all of the Obligations in respect of
the Option Notes.

NOW THEREFORE, the parties hereto agree as follows:

            SECTION 1. Schedule I to the Control Agreement is hereby amended by
deleting such Schedule in its entirety and replacing it with Schedule I as
attached hereto and, for the avoidance of doubt, the Collateral shall include
the Additional Pledged Security Entitlements.

            SECTION 2. On and after the date of this Amendment (i) each
reference in the Control Agreement to "this Control Agreement", "hereunder",
"hereof" or words of like import referring to the Control Agreement shall mean
and be a reference to the Control Agreement as amended by this Amendment and
(ii) each reference in the Control Agreement to "Schedule I" shall mean and be a
reference to Schedule I as amended by this Amendment.

            SECTION 3. Except for the modifications explicitly set forth in this
Amendment, no other amendments or modifications have been made to the Control
Agreement hereby and all other terms and provisions of the Control Agreement
shall remain in full force and effect.

            SECTION 4. This Amendment may be executed in any number of
counterparts and by different parties hereto in separate counterparts, each of
which when so executed shall be deemed to be an original and all of which taken
together shall constitute one and the same

<PAGE>

agreement. Delivery of an executed counterpart of a signature page to this
Amendment by telecopier shall be effective as delivery of an original executed
counterpart of this Amendment.

            SECTION 5. This Amendment shall be governed by, and construed in
accordance with, the laws of the State of New York.

<PAGE>

            IN WITNESS WHEREOF, the parties hereto have caused this Control
Agreement Amendment to be executed by their respective officers thereunto duly
authorized, as of the date first above written.

                                Pledgor:

                                THORATEC CORPORATION

                                By: /s/ D. Keith Grossman
                                    ----------------------------------------
                                Name: D. Keith Grossman
                                Title: President and Chief Executive Officer

                                Trustee:

                                U.S. BANK NATIONAL ASSOCIATION

                                By: /s/ Paula M. Oswald
                                    --------------------------------------------
                                Name: Paula M. Oswald
                                Title: Vice President

                                Account Holder:

                                U.S. BANK NATIONAL ASSOCIATION

                                By: /s/ Paula M. Oswald
                                    --------------------------------------------
                                Name: Paula M. Oswald
                                Title: Vice President

              [Singnature page to the Control Agreement Amendment]

<PAGE>

                                                                       EXHIBIT A

                     Additional Pledged Security Amendment

<TABLE>
<CAPTION>
Par Amount                 Maturity Date                     CUSIP No.
----------              -----------------                   ----------
<S>                     <C>                                 <C>
  213,000               November 15, 2004                   912833FV7

  223,000                  May 15, 2005                     912833FW5

  223,000               November 15, 2005                   912833FX3

  223,000                  May 15, 2006                     912833FY1

  223,000               November 15, 2006                   912833FZ8

  223,000                  May 15, 2007                     912833GA2
</TABLE>

                                    Exhibit A

<PAGE>

                                   SCHEDULE I

                            Pledged Financial Assets

                      Initial Pledged Security Entitlements

<TABLE>
<CAPTION>
Par Amount                Maturity Date                      CUSIP No.
----------              -----------------                   ---------
<S>                     <C>                                 <C>
1,419,000               November 15, 2004                   912833FV7

1,485,000                  May 15, 2005                     912833FW5

1,485,000               November 15, 2005                   912833FX3

1,485,000                  May 15, 2006                     912833FY1

1,485,000               November 15, 2006                   912833FZ8

1,485,000                  May 15, 2007                     912833GA2
</TABLE>

                    Additional Pledged Security Entitlements

<TABLE>
<CAPTION>
<S>                     <C>                                 <C>
 213,000                November 15, 2004                   912833FV7

 223,000                   May 15, 2005                     912833FW5

 223,000                November 15, 2005                   912833FX3

 223,000                   May 15, 2006                     912833FY1

 223,000                November 15, 2006                   912833FZ8

 223,000                   May 15, 2007                     912833GA2
</TABLE>

                                   Schedule I

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