Exhibit 8.1

[Letterhead of Skadden, Arps, Slate, Meagher & Flom LLP]

April 30, 2008

American Capital Agency Corp.

2 Bethesda Metro Center, 14^th Floor

Bethesda, MD 20814

Re: Certain United States Federal Income Tax Matters

Ladies and Gentlemen:

You have requested our opinion concerning certain United States Federal income tax considerations in connection with the offering (the “Offering”) by American Capital Agency Corp., a Delaware corporation (“ACAC”), of shares of ACAC common stock, $0.01 par value per share (“Common Stock”), pursuant to a registration statement on Form S-11 (Reg. No. 333-149167) (the “Registration Statement”), Amendment No. 3 of which was filed with the Securities and Exchange Commission (the “Commission”) on April 28, 2008. We have acted as tax counsel to ACAC in connection with the Offering, and have participated in the preparation of the Registration Statement and certain other documents.

In connection with this opinion, we have examined originals or copies, certified or otherwise identified to our satisfaction, of the Registration Statement and such other documentation and information provided to us by you as we have deemed necessary or appropriate as a basis for the opinion set forth herein. In addition, you have provided us with, and we are relying upon, a certificate containing certain factual statements, factual representations and covenants of officers of ACAC (the “Officers’ Certificate”) relating to, among other things, the actual and proposed operations of ACAC and the entities in which it holds, or has held, a direct or indirect interest (collectively, the “Company”). For purposes of our opinion, we have not independently verified the facts, statements, representations and covenants set forth in the Officers’ Certificate, the Registration Statement, or in any other document. In particular, we note that the Company may engage in transactions in connection with which we have not provided legal advice, and have not reviewed, and of which we may be unaware. Consequently, we have relied on your representation that the facts, statements, representations, and covenants presented in the Officers’ Certificate, the Registration Statement, and other documents, or otherwise furnished to us, accurately and completely describe all material facts relevant to our opinion. We have assumed that all such facts, statements, representations and covenants are true without regard to any qualification as

American Capital Agency Corp.

April 30, 2008

Page 2

to knowledge, belief, or intent. Our opinion is conditioned on the continuing accuracy and completeness of such facts, statements, representations and covenants. We are not aware of any facts inconsistent with the statements in the Officers’ Certificate. Any material change or inaccuracy in the facts, statements, representations, and covenants referred to, set forth, or assumed herein or in the Officers’ Certificate may affect our conclusions set forth herein.

In our review of certain documents in connection with our opinion as expressed below, we have assumed the legal capacity of all natural persons, the genuineness of all signatures, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as certified, conformed, photostatic, or electronic copies, and the authenticity of the originals of such copies. Where documents have been provided to us in draft form, we have assumed that the final executed versions of such documents will not differ materially from such drafts.

Our opinion is also based on the correctness of the following assumptions: (i) ACAC and each of the entities comprising the Company has been and will continue to be operated in accordance with the laws of the jurisdictions in which it was formed and in the manner described in the relevant organizational documents, (ii) there will be no changes in the applicable laws of the State of Delaware or of any other jurisdiction under the laws of which any of the entities comprising the Company have been formed, and (iii) each of the written agreements to which the Company is a party has been and will be implemented, construed and enforced in accordance with its terms.

In rendering our opinion, we have considered and relied upon the Internal Revenue Code of 1986, as amended (the “Code”), the regulations promulgated thereunder (“Regulations”), administrative rulings and other Treasury interpretations of the Code and the Regulations by the courts and the Internal Revenue Service (“IRS”), all as they exist at the date hereof. It should be noted that the Code, Regulations, judicial decisions, and administrative interpretations are subject to change at any time and, in some circumstances, with retroactive effect. A material change that is made after the date hereof in any of the foregoing bases for our opinion could affect our conclusions set forth herein. In this regard, an opinion of counsel with respect to an issue represents counsel’s best judgment as to the outcome on the merits with respect to such issue, is not binding on the IRS or the courts, and is not a guarantee that the IRS will not assert a contrary position with respect to such issue or that a court will not sustain such a position if asserted by the IRS.

We express no opinion as to the laws of any jurisdiction other than the federal laws of the United States. We express no opinion on any issue relating to ACAC or any investment therein, other than as expressly stated herein.

Based on and subject to the foregoing, we are of the opinion that:

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April 30, 2008

Page 3

1.    Commencing with ACAC’s initial taxable year ending on December 31, 2008, ACAC has been organized in conformity with the requirements for qualification and taxation as a real estate investment trust (a “REIT”) under the Code, and its proposed method of operation will enable it to meet the requirements for qualification and taxation as a REIT. As noted in the Registration Statement, ACAC’s qualification and taxation as a REIT depend upon its ability to meet, through actual operating results, certain requirements relating to the sources of its income, the nature of its assets, its distribution levels and the diversity of its stock ownership, and various other qualification tests imposed under the Code, the results of which are not reviewed by us. Accordingly, no assurance can be given that the actual results of ACAC’s operations for any one taxable year will satisfy the requirements for taxation as a REIT under the Code.

2.    Although the discussion set forth in the Registration Statement under the heading “Federal Income Tax Considerations” does not purport to discuss all possible United States Federal income tax consequences of the ownership and disposition of Common Stock, such discussion, though general in nature, constitutes, in all material respects, a fair and accurate summary under current law of the material United States Federal income tax consequences of the ownership and disposition of Common Stock of ACAC, subject to the qualifications set forth therein. The United States Federal income tax consequences of the ownership and disposition of such Common Stock by an investor will depend upon that holder’s particular situation, and we express no opinion as to the completeness of the discussion set forth in “Federal Income Tax Considerations” as applied to any particular holder.

This opinion is furnished to you in connection with the Offering. We consent to the filing of this opinion as an exhibit to the Registration Statement and to the reference to Skadden, Arps, Slate, Meagher & Flom LLP under the headings “Risk Factors,” “Federal Income Tax Considerations,” and “Legal Matters” in the Registration Statement. In giving this consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act of 1933, as amended, or the rules and regulations of the Commission. This opinion is expressed as of the date hereof, and we are under no obligation to supplement or revise our opinion to reflect any legal developments or factual matters arising subsequent to the date hereof, or the impact of any information, document, certificate, record, statement, representation, covenant, or assumption relied upon herein that becomes incorrect or untrue.

Very truly yours,

/s/ Skadden, Arps, Slate, Meagher & Flom LLP