William S. Sessions, new director of the Federal Bureau of Investigation, said yesterday that he will order a review of FBI methods of conducting background investigations after the bureau's failure to discover that Supreme Court nominee Douglas H. Ginsburg had used marijuana.

In his first meeting with reporters since he was sworn in last week, Sessions also said the White House had asked the FBI to conduct the Ginsburg investigation in only five days, even though an average FBI background check takes 25 days.

"That is not to make any excuse," he added.

Ginsburg withdrew his nomination Saturday after admitting Thursday that he had smoked marijuana as recently as 1979 while a Harvard Law School professor.

Sessions said he is concerned that some persons interviewed about Ginsburg may have lied to FBI agents doing the background investigation. If it is determined that anyone lied, Sessions said, the evidence will be forwarded to the Justice Department for possible prosecution.

"You're not entitled to lie to a federal investigator," he said.

Deliberately giving a false statement to an FBI agent is a felony punishable by up to five years in prison and $250,000 in fines. Although the law has not been commonly used, Sessions said he believes it deserves new attention.

"I would have to say that, because so much reliance is placed on what the bureau reports, we will simply have to look at this again," he said. "You rely upon the character of the people you're talking to."

Harvard law Prof. Hal Scott, filmed in a television report last Thursday saying he knew of an instance in which Ginsburg used marijuana, said Friday that he had not mentioned it during his FBI interview because "I had forgotten about this until yesterday."

Sessions said he does not know exactly how questions were phrased in background interviews, but FBI sources have said agents are given a list of questions to use when interviewing family members, friends and colleagues. The list includes questions about subversive activities, alcohol abuse and use of illegal drugs.

The nominee is generally not questioned personally by the FBI, and no drug-related question is on the questionnaires that a nominee must answer and sign. Sessions said that is an oversight that probably should be changed.

Floyd Clark, an FBI assistant director, said the bureau began a full field investigation of Ginsburg a week ago at the request of the White House. It was the fifth FBI background investigation of Ginsburg since he went to the Justice Department in 1983, and 143 interviews were conducted about him. Clark said an internal review is under way.

Sessions also said he does not "want FBI agents who have violated the law in using controlled substances. They should apply somewhere else."

Under current FBI policy, past use of marijuana does not automatically disqualify a person seeking to be an FBI agent. An FBI spokesman said that such matters are handled case by case and that marijuana use is disregarded if it was experimental, infrequent and in the distant past.

Asked whether he believes it should be a priority for the FBI to probe individuals' marijuana use, Sessions said he does not consider drug laws to be "trivial, inconsequential or irrelevant."

"Somebody else may make that judgment, but I don't think it's mine to make . . . . If there's . . . a violation of the law, then it's the bureau's responsibility to inquire . . . . " he said.

In background checks, Sessions said, "I think it's appropriate to ask anybody, 'Do you know of any violation of the law?' That includes the use of controlled substances."

Once the FBI turns up such information, he said, the White House must decide whether to press a nomination. "That's not my business. I am not a prosecutor; I'm an investigator," he said.