On September 1, two psychologists will begin an unusual new training program at Walter Reed Army Medical Center that may provoke the fiercest skirmish so far in the bitter turf fight between psychologists and psychiatrists.

At issue in the experiment mandated by Congress is whether clinical psychologists with additional medical training can safely prescribe drugs, one of the most cherished perquisites of medicine and one of the chief distinctions between psychiatrists, who are medical doctors, and psychologists, who are not.

"If psychologists start prescribing medication, then what is a psychiatrist?" asked E. Fuller Torrey, a Washington psychiatrist who supports extending prescription privileges to psychologists. "The American Psychiatric Association correctly perceives that this is a foot in the door for psychologists."

Officials of both the APA, which represents the nation's 36,000 psychiatrists, and the American Medical Association say they are opposed to letting psychologists prescribe drugs, even with additional training in pharmacology. "To think of pharmacology without anatomy, physiology and biochemistry says that all the things we do to create doctors in this country doesn't mean a thing," said Melvin Sabshin, medical director of the Washington-based APA.

Physicians argue that the sophistication required to properly prescribe psychotropic drugs that alter brain chemistry requires a medical degree that provides firm grounding in the basic sciences augmented by several years of clinical training. "It scares me to think of psychologists prescribing," said AMA president C. John Tupper. "I don't want to be treated that way, and I don't want my family to be treated that way.

But many psychologists see the issue far differently. They argue that they are often better trained to treat mental disorders than many of the general practice physicians who now care for and prescribe drugs for patients with psychiatric problems. In addition, they note that psychologists are performing much of the research on new psychiatric drugs.

"We have psychologists who are very well trained in diagnosing and treating mental illness," says Bryant Welch, executive director for clinical practice at the American Psychological Association, which has 105,000 members. "But the one thing that they have not been trained or licensed to do is to prescribe psychoactive drugs."

Psychologists aren't the only ones seeking prescription privileges. In some jursidictions, including the District, registered nurses with advanced degrees -- among them midwives, nurse practitioners and nurse anesthetists -- can prescribe some medications on a limited basis. So can physicians' assistants, optometrists, podiatrists and pharmacists. Although physicians' organizations generally have opposed the extension of prescription privileges to these groups, some doctors contend that nurses and physicians' assistants have more of a grounding in the basic sciences than do psychologists.

Some who favor extending privileges to psychologists argue that it would help compensate for the scarcity of psychiatrists in rural areas and at state mental hospitals. "The consequences of this psychiatric manpower shortage for the care of the seriously mentally ill are predictable and disastrous," said Torrey, who has written extensively on the plight of the homeless mentally ill. "Why not give those psychologists who are interested a training course in medication utilization and let them prescribe?"

That's a question that Sen. Daniel K. Inouye (D-Hawaii), chairman of the Senate subcommittee on defense appropriations, has been asking as well. Inouye, whose administrative assistant Patrick H. DeLeon is a clinical psychologist, a member of the American Psychological Association's board of directors and author of several articles advocating prescription privileges for psychologists, first proposed the upcoming demonstration project in 1987.

After months of study, in September, 1988, Inouye secured congressional approval for a mandate ordering that the Defense Department begin the demonstration project. Despite protests from the American Psychiatric Association, the Army Surgeon General's office began drafting a proposal to implement it. Last February, a committee composed of representatives from both the psychological and the psychiatric associations met to discuss it in greater detail.

But last month, the plan was still in its preliminary stages when Assistant Secretary of Defense Enrique Mendez Jr. sent a memo to the assistant secretary of the Army noting "Senator Inouye's intense interest in an early response." Mendez ordered that the Army begin the program on September 1. The other branches of the military are not involved in the program.

Last week, Army officials said that they had chosen the two psychologists to train in the pilot program and had tentatively decided on the curriculum: a year-long course that will involve clinical training at Walter Reed and classes in pharmacology and biochemistry at the Army's school for physicians' assistants in San Antonio.

Once they complete the program, the two psychologists, whose names the Army has not released, will be tested by an independent board of experts before they will be allowed to prescribe any medication without supervision.

"The idea has merit," said Col. Joe Fagan, a psychiatrist who helped design the program for the Army Surgeon General's Office. "If we could theoretically come up with a program to allow psychologists to expand their practice without going back for eight years of school, that would be good."

But the Army is "really not prepared to take on a mission that should be relegated to graduate schools of psychology," he said. "

The Indian Health Service, which operates a network of 50 hospitals and 139 health centers on Indian reservations, granted limited prescription privileges in recent years to psychologists. The program was suspended in December 1988 by the Public Health Service following criticism that Native Americans were getting substandard treatment.

The Army's program is being closely watched by supporters and opponents who say it could lay the groundwork for states to revise laws governing who can prescribe drugs.

The quest for prescription privileges is the latest in a continuing clash between psychiatry and psychology. Since 1945, when the Connecticut legislature passed the nation's first law to license psychologists, the two professions have often been at odds. Psychiatrists have watched as psychologists, later joined by psychiatric social workers, marriage and family therapists and psychiatric nurses have competed with them for patients.

More recently, California psychologists won a ruling from the state supreme court allowing them to admit and treat patients in general hospitals. The Hawaii state legislature passed a resolution in April asking a task force to study the issue of prescription privileges for psychologists.

Both professions are based on "talk therapy," the idea that by discussing feelings and past experiences, it is possible to resolve emotional problems. Clinical psychologists spend an average of five years in graduate school to earn a doctorate and then train for another year or two with a more senior psychologist before establishing a practice.

Psychiatrists complete four years of training in addition to four years of medical school. Like psychologists, they must pass state licensing exams.

The ultimate decision of whether psychologists can legally prescribe drugs resides with state legislatures. Even if the law were to be changed to enable psychologists with special pharmacological training to prescribe, that would be an impossibility for many who would have to suspend their practices while they went back to school.

If psychologists were permitted to prescribe, said Raymond Folen, a clinical psychologist practicing in Hawaii, it is likely he would opt for the additional training. "But financially," he said, "it might be difficult for me."