Dr. Peter G. Bourne, President Carter's chief adviser on health and drug abuse, was placed on leave of absence at his own request last night pending the outcome of a criminal investigation of a drug prescription for a fictitious person made out and signed by Bourne.

White House press secretary Jody Powell, who announced the decision at 7:45 p.m., said the president agreed with the action. Powell said he assumed Bourne's leave of absence would be with pay.

In a three-page statement issued by the White House, Bourne defended his use of a false name on the prescription, which he said was intended for the use of one of his aides.

"I took what I believed to be legitimate precautions to protect the confidentiality of the individual involved," Bourne said. "The use of a pseudonym on a prescription is one way of protecting confidentiality."

The drug that Bourne prescribed for his aide is Quaalude, a tightly restricted drug used at a physician's direction to produce sleep or sedation. A young woman identified yesterday as a friend of Bourne's aide was arrested July 11 when she tried to fill the prescription at a drugstore in Woodbridge in Prince William County.

Powell said the decision to take a leave of absence was Bourne's alone and that Carter was informed of it after it was made. Powell said a final decision on whether Bourne will return to the White House staff will be a "mutual decision" between Bourne and the White House.

Powell refused to say what the White House would consider a satisfactory resolution of case.

Defending the decision to grant Bourne a leave of absence with pay, Powell said, "it seems to me in the interest of not prejudging the matter that it woudl be somewhat excessive to deprive him of his livelihood during the time he is attempting to deal with it." Powell said he did not know whether Bourne had prescribed drugs for other members of the White House staff.

In his statement, Bourne offered a lengthy defense of his use of the Fictitious name.

"I have consulted legal counsel and believe that what I have done was neither legally or morally wrong," he said.

"I wrote a real prescription to and for use by a real person with a real medical problem. The prescription was written for a resident of the District of Columbia, where I am licensed to practice."

In his defense, Bourne also cited Section 9 of the principles of medical ethics, which he said prevents a doctor from revealing the confidence entrusted to him "in the course of medical attendance."

Bourne said that he used the faluse name because he and his patient "were concerned about the protection of her confidentiality both with regard to her taking medication and to the creation of a record anywhere that she had been treated by a psychiatrist."

The commonwealth's attorney for Prince William County said yesterday that he is considering prosecution of Bourne for the Writing of the prescription to a ciftitious person.

"I have been reading the law," said the prosecutor, Paul B. Ebert, "but I'm not in a position now to say what he (Bourne) would be charged with."

Elbert said that either a misdemeanor or a felony charge is possible, but that before making a decision he would wait about a week for the completion of the county police investigation.

The prescription signed by Bourne was made out to "Sarah Brown," apparently a fictitious person, law enforcement sources said yesterday.

Toby M. Long, 26, a physical theraphy counselor who lives at 1644 33rd St. NW tried to have the prescription filled at a People's Drug Store in Woodbridge on July 11 and was arrested on the spot, according to county police.

She was charged with "feloniously obtaining or attempting to obtain a controlled drug known as Quaalude," according to the arrest warrant.

Quaalude is a potent drug prescribed to induce sleep or sedation. It use is tightly regulated under federal law but it has been gaining popularity among drug users, some of whom believe it has aphrodisiac qualities.

Law enforcement officers have interviewed Bourne twice about the incident, according to informed sources, and Bourne has told thme he made out the prescription for 15 Quaalude tablets to a fictitious person in order to avoid embarrassing the aide for whom the drug was intended. That person was identified as a woman who works in his office, according to these sources.

Officials at the American Medical Association and the D.C. Medical Association said yesterday that the use of a fictitious name in making out a prescription would be unprofessional conduct.

Immediately following her arrest, Long was released on $3,000 bond. A preliminary hearing is scheduled for her in Prince William General District Court on Sept. 19.

According to her attorney, David Sher, Long was blindly carrying out a favor for her friend and former college roommate in trying to get the presciption filled.

Law enforcement sources identified the friend as Ellen Metsky, an administrative aide to Bourne.

Metsky could not be reached for comment despite repeated phone calls made to her yesterday.

Long "has never done anything wrong in her whole life and has not done anything wrong now," Sher said late yesterday. He said that although the prescription was made out to Sarah Brown, Long received it from Metsky and believed that Metsky would use the Quaalude. Sher portrayed his client as a close friend of Metsky from college days at Boston University.

Metsky was described yesterday by people who know her as a "sweet" and "naive" woman who worked with Bourne early in the Carter campaign.

The incident began to take on bigger proportions last week when District of Columbia police investigators, who also are looking into the matter, met with the principal U.S. attorney here, Earl J. Silbert, to give him details, including the White House Involvement.

Silbert then informed Deputy Attorney General Benjamin Civiletti about the incident late last week. New Justice Department procedures require that the attorney general's office be notified of any pending investigation into a public figure. These procedures were instituted earlier this year after conflicting reports about whether high administration and Justice Department officials were aware of a federal investigation into Rep. Joshua Eilberg (D-Pa.).

Last Friday, a spokesman for the Justice Department notified presidential press secretary Powell, who was with the Carter entourage in Bonn for an economic summit. It could not be learned when the president learned of the matter.

Powell said last night that Carter at no time had asked Bourne to resign.

Bourne is an English-born psychiatrist who helped plan early strategy for Carter's campaign for the Democratic presidential nomination while also working on drug abuse problems as a Nixon White House staff member.

Presidential press secretary Jody Powell once identified one of Bourne's strong points as his "knack for getting along with a lot of different people." He has been "one of the few people who can go into the Oval Office at any time," according to a White House source.

Bourne came to this country in 1957, enrolled at Emory University in Atlanta, and was graduated from its medical school in 1962. He performed his internship and residency at Harborview Medical Center in Seattle, and at the Stanford University Hospital. He lists his primary specialty as psychiatry with other specialties in neurology and general practice.

He is licensed to practice medicine in Georgia, California, Connecticut, Massachusetts, and the District of Columbia.

Bourne, who had served as a captain in the U.S. Army during the Vietnam War helped found the Vietnam Veterans Against the War during the 1960s and demonstrated in Chicago during the 1968 Democratic National Convention. His opposition to the Vietnam War while working in a Green Beret camp near the Camboidan border as an army research psychiatrist. "I came back totally disenchanted with the war," he has said.

While working on a master's degree in anthropology at Stanford, Bourne worked in San Francisco's Haight-Ashbury free clinic, serving an area where the explosion of teen-age drug use in this country began.

He returned to Emory in 1969 to teach, do research and run a community mental health center. A mutual friend introduced him to Carter, who then was running for governor.

After his election as governor, Carter made Bourne a health and mental health adviser. In 1971, Carter selected Bourne to set up the state's drug-abuse program in 1971.

"I never really viewed myself as a great drug expert," Bourne said in a 1976 interview with Washington Post reporter Stuart Auerbach, "but I suddenly found myself the only person in Georgia who knew anything about the drug programs."

In those first years in government in Georgia, Bourne became close to both Jimmy and Rosalynn Carter through their mutual interest, in drug problems and mental health. Mrs. Carter particularly singled out mental health as her major interest as first lady of the state.

Both Carter and Bourne won high marks from mental health professionals for their achievements in Georgia.

Bourne was hired in 1972 as assistant director of the Nixon White House's Special Action Office for Drug Abuse Prevention, set up by President Nixon to battle the nation's drug problem. Bourne quit two years later because, he said, the Carter presidential campaign was taking up too much of his time.