Still, at Sept. 30, the firm held $710.2 billion in its retained portfolio, vs. $755.3 billion at year-end. It was well within the Treasury Department's requirement to shrink its loan portfolio from a maximum of $900 billion at Dec. 31, to $810 billion a year hence.

"The FHFA has stated its expectation that we will not be a substantial buyer or seller of mortgages for our mortgage-related investments portfolio, except for purchases of seriously delinquent mortgages out of PC trusts," the company said in its quarterly filing.

Fannie Mae hasn't reported third-quarter results yet. However, its retained portfolio moved the opposite way during the first half of 2010 - again, because of put-backs from mortgage trusts.

Fannie's retained portfolio grew 6% through June 30, to $817.8 billion from $772.7 billion at the start of the year. It bought back $170 billion worth of delinquent loans in mortgage-backed securities (MBS) trusts during the first half of the year.

But, meanwhile, both Fannie and Freddie continued to "liquidate" holdings of subprime, Alt-A and other "toxic" mortgage debt by tens of billions of dollars. There are a couple of ways Fannie and Freddie can go about doing so. They can sell related MBS in the market - though there are few buyers, usually offering pennies on the dollar. Additionally, when a property enters foreclosure to become "real-estate owned," or REO, it is also considered liquidated.

At Sept. 30, Freddie Mac had nearly 75,000 of REO properties, worth $7.4 billion - up 19% over the past three months and up 77% year-over-year. At June 30, Fannie Mae had nearly 130,000 single-family properties considered REO, worth $13 billion - more than double the year-ago level.

The trends are telling in several ways.

First, Fannie and Freddie are providing the support needed for the housing market's recovery by acting as a conduit for mortgage finance. But they're also trying to get rid of a huge number of bad loans in a friendlier manner than their banking peers - one with "integrity," as official statements say.

Secondly, Fannie and Freddie are also getting lots of pushback from private investors in the MBS they stand behind. The hundreds of billions' worth of buybacks Fannie and Freddie have accepted make the amount of loans they're pushing onto large servicers like Bank of America ( BAC), JPMorgan Chase ( JPM), Wells Fargo ( WFC) and Citigroup ( C) look like relative chump change.

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