One of the men whose arrest sparked investigations of alleged drug use on Capitol Hill pleaded guilty yesterday in U.S. District Court to two felony narcotics charges.

Troy M. Todd Jr., 23, named by a federal grand jury last November as the manager of a ring that allegedly distributed drugs on Capitol Hill, pleaded guilty to charges of conspiring to distribute cocaine and possession with intent to distribute cocaine.

Judge Thomas F. Hogan, who accepted the plea, indicated he would set sentencing for Todd probably at the end of August or early September. Each charge carries a maximum penalty of 15 years in prison and a fine of $25,000.

Todd and former congressional page Douglas W. Marshall, 27, were first arrested in April 1982 in Marshall's home in Northwest Washington after allegedly selling eight ounces of cocaine to an undercover D.C. police officer.

Their arrests sparked federal and congressional investigations of alleged drug use by some congressmen and Capitol Hill aides. A third man arrested at that time, Robert A. Finkel, 30, who also worked on Capitol Hill, is now in the federal witness protection program.

Marshall pleaded guilty June 20 to one count of conspiracy to distribute cocaine and agreed to testify before a grand jury investigating allegations of drug use on Capitol Hill.

Sources familiar with the investigation said yesterday that Todd has not agreed to cooperate with the government in exchange for the plea.

Todd's lawyers, Stanley M. Dietz and Plato Cacheris, have said that their client was not in a position to lead prosecutors to Capitol Hill figures. Sources familiar with the investigation said there has been no indication that Todd personally supplied drugs to anyone on Capitol Hill.

Todd and Marshall went to Australia shortly before they were indicted here in November. They were arrested by Australian police in January and extradited to Washington in April.

Todd, who was released on a $300,000 surety bond, said after the brief hearing yesterday that he was "relieved" by entering the guilty plea. "I'm trying to put the whole thing behind me," he said, adding: "It's been tough on my family."

So far, only one person has been sentenced after publicly pleading guilty in the case. He is Robert T. Yesh, a former employe in the House doorkeeper's office who pleaded guilty in March to two misdemeanor drug charges and was sentenced in April to one year in prison.

Hogan warned Todd yesterday that failure to appear for sentencing could result in increased penalties.

"Don't worry," Todd replied, "I'll be here on time."