As part of its campaign to encourage financing of inner-city rehabilitation, the Federal National Mortgage Association (Fannie Mae) announced this week that it will begin buying some second mortgages, graduated payment mortgages, home improvement loans and other mortgage variations from lenders.

But for the moment, Fannie Mae said, the new policy applies only to loans on single-family homes in two districts of St. Louis, Mo., where it is launching a pilot project to stimulate inner-city lending.

This marks the first time Fannie Mae, which purchases government-backed and conventional mortgages nationwide, has been in the market for loans such as those for home improvement. The nation's largest buyer of mortgages on the secondary market, Fannie Mae may extend its new buying policy to 12 more areas in St. Louis and other ailing cities, officials said.

They said the decision was made only after a FNMA task force chose two areas of St. Louis, the Shaw and Skinker'DeBaliviere neighborhoods, both of which are characterized by "architecturally important" brick housing dating from the turn of the century. Fannie Mae has also observed that a few young professionals have begun to move back into these areas.

The St. Louis pilot project is aimed at stimulating lending for rehabilitation of housing in those neighborhoods, but Fannie Mae is telling lenders it will repurchase only "solid loans" where little risk has been taken.