Federal investigators pursuing allegations of sexual misconduct and drug use involving members of Congress are about to shut down the sex probe for lack of corroboration, but are continuing to evaluate the possibility of narcotics violations, informed sources said this week.

Meanwhile, veteran Washington lawyer Joseph A. Califano Jr., the newly-appointed special counsel to the House ethics committee, is gearing up his own, independent investigation into all allegations. His investigation is expected to take several months, sources said.

The Justice Department's probe into possible sexual misconduct had centered on allegations from at least three former pages on Capitol Hill that members of Congress had solicited sex from underage pages or had used them to arrange sexual liaisons.

Key sources involved in those allegations have not proved reliable, and FBI agents conducting field investigations have been unable to obtain any evidence that would warrant a criminal prosecution, informed sources said.

One of the pages had failed parts of an FBI lie detector test and had conceded in interviews with The Washington Post that he had exaggerated parts of his allegations. The federal investigation into possible sexual misconduct is expected to be closed shortly, possibly as soon as this week, the sources said.

At the same time, the Justice Department believes that it is too early to evaluate charges of drug abuse -- including allegations that a drug ring involving Capitol Hill aides distributed cocaine to members of Congress -- and that more work has to be done to determine the scope of possible criminal involvement. Agents for the Drug Enforcement Administration and Metropolitan Police Department are continuing to look into that for the Justice Department.

The drug investigation stems from months of undercover work around Capitol Hill by DEA agents and metropolitan police detectives. Last year Rep. Robert K. Dornan (R.-Calif.) permitted a D.C. policeman to claim he was a Dornan aide while conducting an investigation into possible cocaine distribution.

The allegations that Congressmen might be exploiting some of the 100 teen-age pages who worked for them caused a furor when given extensive media attention in June. The attention has subsided substantially as investigators have been unable to find evidence to back up the allegations.

A special House commission has been established by House speaker Thomas P. O'Neill Jr. (D.-Mass.) to consider reform of the page system, in which teenagers -- some as young as 14 years -- come to Washington to run errands for Congress. The commission, chaired by Rep. Bill Alexander (D.-Ark), is expected to hold a press conference today at which it will release its recommendations. The concern in Congress is that the pages live without much supervision, often in their own apartments, and that if the system is to be continued, they should be housed in a more restricted environment.

Califano, appointed special counsel to the House ethics committee late last month, has been given a mandate to look into all the allegations, which O'Neill has called "an affront to Congress, to the nation it serves, and to basic human decency."

One source said Califano's staff has talked about completing their work within three months, although delicate maneuvering on Califano's side to avoid intereference with the Justice Department's investigation of drug abuse may slow things down. Califano has talked with Attorney General William French Smith and has met with FBI director William Webster to discuss his relationship with the Justice Department and federal investigators, sources have said.

House resolution 518, passed by the House last July 13 by a vote of 407 to 1, gives Califano the authority to report to the Justice Department or other authorities "any substantial evidence" or wrongdoing by a House member or employe that he uncovers during his investigation. Otherwise, Califano's investigation for the ethics committee could lead to recommendations for internal disciplinary measures, such as censure or expulsion.

Over the past 10 days, sources said, Califano's office has been involved with recruiting staff and establishing relationships with federal investigators. Califano and his staff have set up shop on the fifth floor of a drab federal office building, known as House Annex No.2, where a corner group of offices has been redecorated and refurnished for the new arrivals. A Capitol Police officer is planted at a desk outside Califano's office and visitors are not welcome.

Califano has appointed a deputy special counsel, Richard Cotton, a partner at Califano's Washington law firm, Califano, Ross & Heineman, and a temporary executive assistant, John Hughes. Cotton worked at the Department of Health, Education and Welfare, which Califano ran during the Carter administration. Hughes is a first-year law student working in the special counsel's office for five weeks.

Sources have said that Califano's office is concentrating now on hiring a full-time senior attorney, with extensive investigative background, to be a key assistant. The leading candidate for that job is Hamilton P. Fox III, a former assistant U.S. Attorney and member of the Watergate prosecution team, who is now in private practice, according to sources. Fox also served as deputy chief of the organized crime section of the Justice Department.

Califano's office has taken on three investigators from the ethics committee, which had investigated the sex and drug abuse allegations before Califano took over, and has hired two more investigators, including a highly respected detective from the New York police department. According to informed sources, staff members have said that they expect the investigators to be on board for several months.

Sources said that Califano's office expects to be up to full speed by the end of this week, and will concentrate first on absorbing information from previous investigations into the charges.

Through the ethics committee, formally known as the House Committee on Standards of Official Conduct, Califano has the power to issue subpoenas for documents, such as bank or telephone records, or testimony, either before the ethics committee or in the form of sworn depositions.

The attorney representing former page Leroy Williams, who alleged first hand knowledge of sexual misconduct by members of Congress, said yesterday that his client was informed last week by one of Califano's investigators that he should expect to come to Washington within a week to 10 days.

Califano has declined all requests for interviews, as has his staff. They cite a House rule that says no member of the committee or staff can disclose information of any kind.