John O'Brian finished medical school at the University of Vermont, completed his internship, residency and chief residency at St. Vincent's Hospital in New York, and received a fellowship from Harvard University's Thorndike Memorial Laboratory. Then the Navy told him it was time.

The Navy said O'Brian had had enough deferments under the Berry Plan, a wartime plan that allowed doctors to gain expertise in specialty areas while putting off service. So O'Brian was obligated to serve for two years.

Those two years ended in 1976. Almost 10 years later, enjoying a length of stay at the same hospital, which few other military doctors have had, Capt. O'Brian is still on staff at Bethesda Naval Hospital.

"In terms of motivation, there are as many motivations for staying as there are people," O'Brian said. "In this facility, I've been able to accomplish every professional goal that I've had . . . . I like the sense of an organization around you."

O'Brian, 42, is head of the endocrinology department and an associate professor at the Uniformed Services University of the Health Sciences, the nation's only military medical school, which opened in 1976.

Unlike many doctors who are transferred every few years, moves that the Defense Department says are necessary to ensure readiness, O'Brian served three years at Bethesda and then was made head of the department.

"I've been fortunate. It's happened to me de facto. I don't have to worry . . . . In all candor, everyone has specific objectives. Had I not been here, I might well have reacted differently. Fortunately, the chronology was right and my productivity level was acceptable."

O'Brian estimates that he makes between $60,000 and $70,000 and receives service benefits that are worth $15,000. He has no malpractice insurance or operating costs.

"I would probably need a 75 percent increase in my pay to have the same on the outside that I do here," O'Brian said. "If you wanted to do endocrinology and nothing but that, you'd probably starve out there."

Last month, anesthesiologist Robert Brunsvold called an end to a 10 1/2-year career in the Army. He had joined in February 1973 under the auspices of the Health Professions Scholarship Program, a program that would get him through the last two years of costly medical school. He never thought it would lead to a decade of working in the military.

"When I look back over the decision of coming into the military, I'm extremely happy I did it. It's been very good to me. The training was excellent and I've had a very strong clinical experience. That's probably the best thing about military medicine. If I could do it again, I would. But . . . .

"There are a lot of reasons for me to go. I want to practice in the Midwest. All of my family is in the Midwest. I've maxed the pay scale at $68,000, and I don't think that is going to get me by. I can live on that myself, but when I have four kids who want to go to college, I can't.

"And then there's the dissatisfaction I have with the military. We're short of nurses, corpsmen; we don't have enough people to run our operating room or staff our hospital. We shouldn't have that kind of problem. Military medicine should be the best. And we're not. We don't have enough people.

"I've also moved seven times in the past 16 years. I just don't want to do that anymore."

Brunsvold, who practiced at Walter Reed Medical Center, already has bought a new home in Fargo, N.D. Unabashedly enthusiastic during his interview at his old office, he took out some color slides of the place, turned on a projector and beamed the picture onto his wall.

"I plan to join the Reserves. I'm not exactly sure why. I guess I just can't walk away from it."

Air Force 2nd Lt. Michael Edwards is a second-year medical student at the Uniformed Services University of the Health Sciences. The son of a career Air Force man, Edwards knows he will have an 11-year commitment to the Air Force after the free medical school is over. That is not too much to ask, said Edwards.

"I'm going to get out and practice medicine without ever having to wonder if the patient can afford the service . . . . And I know what it's like being in an Air Force family. I was in five or six different grade schools as a kid. I weighed all that out and thought: 'Do I want to uproot my wife and kids every few years?' I felt I had no negative experience from what I had done.

"But I did think it would be nice to leave my medical education and not have to have a $100,000 debt."