Going to a doctor's office 20 years from now will be a far different experience than the typical physician visit today.

"Essentially there will be a lot more physicians," says Dr. Alvin Tarlov, president of the Henry J. Kaiser Family Foundation in California. Projections by the Graduate Medical Education National Advisory Committee (GMENAC) point to 645,000 practicing physicians in the year 2000 -- some 145,000 more doctors than will be required.

Chances are the patient of the future will visit the doctor at a health maintenance organization or other prepaid health plan, rather than at an individual doctor's office or a group practice. The GMENAC report estimated that 100 million Americans will be enrolled in HMOs during the next 15 years -- an increase of 80 million men and women.

Physicians of the future are more likely to be women. Since the 1970s the number of female doctors has increased from less than 5 percent to some 30 percent. "That represents a very, very interesting change," says Tarlov, who chaired the GMENAC and updates the 1980 report in the new issue of the journal Health Affairs.

But even fewer physicians in the future are likely to be from lower-income groups. Reductions in federally financed school loans mean that the rising price of medical school is out of the reach of many students. A new survey by the American Association of Medical Colleges finds that medical students "are tending to come from more affluent social strata and large urban environments," says AAMC's Dr. August Swanson.

Today's medical students are also "less interested" in careers as family or pediatric physicians.

They are also apt to be more heavily in debt by the time they complete school because of the loss of many federally financed medical school loans, the AAMC report says. The average debt for all 1984 graduates was about $23,000, while minority student graduates owed $25,000.

Whether this will have a chilling effect on the number of minorities who apply to medical school is not clear, but in 1985, for the first time in 10 years, the number of minorities who applied to medical schools decreased, the AAMC reports.

These future doctors will also work as salaried employes of a health plan, hospital or clinic, rather than as solo practitioners or members of a group practice. For physicians long accustomed to practicing alone or with a small group of colleagues, there will also be significant changes. The doctor of the future is likely to be a salaried employe, subject to the policies and bottom-line figures set by non-medical boards of directors.

The result will be a major shift in the ways doctors are organized. Today, physicians are united by the powerful American Medical Association. In the next 20 years, experts predict, they will be linked by labor organizations, particularly unions.

"We're headed toward unionization with the development of the corporatization of medicine," says Dr. Peter Orris, an attending physician at Cook County Hospital in Illinois and a member of Physician's Forum, an organization that looks at issues in medicine. As more and more companies run hospitals and clinics, Orris predicts, "a greater and greater percentage of doctors will need to adopt a trade union form of organization to secure stability of employment and to assure the ability to provide quality health care."

"I don't know whether physicians will actually call their groups unions or not," adds Tarlov. "But for sure, physicians are organizing themselves in a way to meet the present demands of care."

Already, this trend is occurring in health maintenance organizations (HMOs), the prepaid health plans gaining in popularity in the United States. Tarlov predicts that "these collectives of doctors can have the same impact as unions." Whether they will be used for collective bargaining, setting wages or strike tactics, Tarlov says, remains unclear. "But organization of doctors is becoming a reality," he says.

With growing numbers of physicians practicing, experts say, competition will increase. Salaries may even drop, particularly for the high-priced specialities. "What will happen to doctors' salaries?" asks Dr. Milton Terris, president of the National Health Association for Public Health Policy and editor of the Journal of Public Health Policy. "I think that there will be a lot more equality among physicians, so that family practice doctors and pediatricians will do better and surgical specialities will not do as well."

Doctors are also likely to be working for large conglomerates, which will control all facets of health care, from medical clinics and teaching hospitals to nursing homes. Evidence for this trend comes from the move of major hospital chains such as Humana and Hospital Corp. of American into the medical clinic business, experts say.

Some teaching hospitals are also branching out to become health care conglomerates, reports Jay Toole, a partner and specialist in the health care industry for Arthur Andersen. First among the entrants is the Mayo Clinic of Rochester, Minn., which is now setting up satellite clinics in Florida. "This is the beginning of a trend," Toole says. "These major organizations are going nationwide over a period of time."

The idea behind these "health conglomerates" is to control the whole health care delivery system. "If you get a fixed price for treating a patient, and you can't control the physician piece or the hospital piece of the pie, you are running a financial risk," Toole says. "You're going to see combination of delivery systems in the country getting into this."

"This will give us a nationalized, privately held health care system in which medical decisions are made in the boardroom based on the profit line, not on patients," predicts Cook County Hospital's Orris. "This country is trying a novel experiment that no other country has tried. We're trying to organize a national health service under private services and direct it toward profits."

The switch toward being an employe rather than the boss will probably result in some scheduling changes for physician and patient. "Some people call it the 7-Eleven approach," says Dr. Beverly C. Morgan, chair of the department of pediatrics at the University of California Medical School at Irvine and a member of the American Academy of Pediatrics Committee on Manpower. "You're in and you're out. You work set hours and go home and forget about it. Doctors in the future will be working fewer hours."

Better technology and greater competition will all translate to more free time for physicians, writes Kaiser Family Foundation's Tarlov in an upcoming issue of the journal Health Affairs. In turn, that shift should benefit consumers, he says, since doctors will have more time for their patients. They will also have additional time for continuing medical education, for teaching and for caring for the poor.

Patients also may play a more active role in their own treatment. A 1984 report by Arthur Andersen for the American College of Hospital Administrators projects that by 1990, some 36 percent of patients will "usually obtain second physician opinions regarding surgery or major therapeutic treatment," compared with just 25 percent in 1984.

In addition, the report suggests that in 1990:

More than 70 percent of patients will at least occasionally select the hospital they go to, while 8 percent of patients will "usually" select the hospital. By comparison, only 11 percent of patients "occasionally" selected the hospital they were admitted to in 1984.

About eight of every 10 patients will occasionally reject their physician's advice for diagnostic testing, compared with just 9 percent in 1984.

About 17 percent of patients will usually select their doctor based on where he or she has admitting privileges to hospitals, and 74 percent will occasionally select the doctor on that basis. In 1984, only 11 percent of patients usually chose a doctor based on admitting privileges, while 66 percent occasionally did.