A federal grand jury investigating the House Post Office yesterday indicted former congressional aide Gerald W. Weaver II on drug, obstruction of justice and conspiracy charges.

Weaver, 37, is a former assistant to Rep. Joe Kolter (D-Pa.), who also has been identified as a subject of the federal investigation. Kolter was defeated in a primary earlier this year.

The grand jury said Weaver cashed $2,800 in checks at the Post Office to buy cocaine but lied and said the cash was used to buy stamps.

Weaver is the first congressional staff member to be charged in the probe of mismanagement and illegal practices at the House Post Office. Five people, all former employees of the House Post Office, have pleaded guilty to charges arising from the investigation.

Weaver's lawyer, Russell F. Canan, said yesterday that his client, a self-employed lobbyist, "strongly denies these charges."

The indictment charged that Weaver gave the grand jury a phony letter from a business colleague that stated Weaver had purchased more than $2,800 in stamps. The indictment said, Weaver cashed the checks at the House Post Office between September 1989 and May 1991 to purchase cocaine from a House Post Office employee, Wendell Magruder.

Magruder has left the post office and has pleaded guilty to embezzlement and drug charges and promised to cooperate with the investigation.

Weaver was charged in the 14-count indictment with possession and distribution of cocaine, conspiring to possess cocaine, obstruction of justice, tampering with a witness and conspiring to commit wire fraud. If convicted on all charges, Weaver could face a maximum sentence of 49 years and fines up to $500,000. Federal sentencing guidelines could provide for a substantially shorter sentence.

The indictment alleged that Weaver attempted to persuade Magruder not to cooperate with the grand jury by promising him a job in Weaver's lobbying firm, a loan and assistance in finding a lawyer. In addition, the indictment charged Weaver with conspiring with Magruder and others to stage a theft of a car owned by Weaver's wife and then file a false automobile insurance claim.

Magruder admitted embezzling more than $13,000 from his stamp drawer at the House Post Office, concealing a material fact from federal investigators and conspiring to distribute drugs. In return, prosecutors agreed to drop a fourth charge, distribution of cocaine, which carries a maximum 25-year sentence.

Earlier this year, the grand jury subpoenaed the expense records from 1986 to April 1992 for Kolter, House Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.) and Rep. Austin J. Murphy (D-Pa.).

The three have dismissed the probe as a "political witch hunt" directed by U.S. Attorney Jay B. Stephens, a Republican appointee.

Kolter could not be reached at his congressional offices here or in Pennsylvania.