WASHINGTON (AP) _ A federal grand jury investigating the House Post Office has subpoenaed four aides to Rep. Dan Rostenkowski, including three staffers of his Ways and Means Committee, a congressional source said Thursday.

The subpoenas, requiring personal appearances Jan. 7, are the first served on staffers of the tax writing committee.

Subpoenas were issued to Robert Leonard, committee chief counsel and staff director; press secretary James Jaffe and Charles J. Mellody, assistant to the chairman, said the source, speaking only on condition of anonymity.

Also subpoenaed was Chris Kanthak, a receptionist in Rostenkowski's congressional office.

Two other House members have acknowledged they also are under investigation in the post office probe.

The grand jury reportedly is trying to determine whether Rostenkowski, D- Ill., or others - including their campaigns - obtained money illegally from the post office, through transactions disguised as stamp purchases.

Sources said the investigation lately has been following the trail of money spent by Rostenkowski's campaign organization and political action committee.

Another recent issue involves possible attempts by those under investigation to obstruct the grand jury, according to public comments of U.S. Attorney Jay B. Stephens.