WASHINGTON -- Fannie Mae and Freddie Mac, the giant mortgage concerns, are coming under fresh regulatory scrutiny in the wake of the Orange County derivatives debacle.

The Office of Federal Housing Enterprise Oversight, which monitors the safety and soundness of the two secondary-market players, is asking for a detailed accounting of how Fannie and Freddie ensure that the risks of their derivatives are disclosed to investors.

So-called structured notes issued by the agencies have figured in several investment fund losses this year, including the high-profile California case.

Though the notes make up only a small portion of the agencies' funding base, experts say that problems with the instruments could broadly affect the agencies' financing efforts. Already, the yields on agency debt have risen somewhat relative to Treasury securities, analysts say.

Aida Alvarez, director of the oversight office, made her concerns known in a Dec. 22 letter to the agencies.

The letter asked the agencies for information on the written and oral disclosure they require dealers to make on the interest rate risk posed by Fannie and Freddie securities.

The regulator also wants to know what rules the agencies require dealers to follow in disclosing the lack of government backing for the securities. Agency debt carries what market participants call "implicit," but not actual, government guarantees.

Thomas O'Donnell, an analyst with Smith Barney said he thought it possible that Fannie and Freddie would be asked to play a more active role in monitoring dealer practices.

The agencies may be required to provide "a little more description of what would happen to the security under different interest rate scenarios," another analyst said.

Fannie Mae spokesman David Jeffers called the letter "a routine request from our regulator," adding that the agency would respond "in our normal expeditious fashion."

A Freddie Mac spokeswoman said her agency did not have a comment on the letter yet.

In the letter, Ms. Alvarez said her office is concerned that publicity surrounding the recent losses could "obscure the legitimate uses of structured debt and other derivatives by investors."

She added: "Continued reports of problems in these areas could also have detrimental effects on the market for all GSE securities."

Thomas Stanton, a Washington attorney who has studied the agencies closely, said the regulatory office appeared to be studying whether agency practices have contributed to derivatives losses.

But Eugene Carlson, director of public affairs at the oversight office, said his agency "has no reason to believe that there is a problem" in agency practices.

"This is fact finding, pure and simple," Mr. Carlson said.

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