For decades, the typical American doctor was in "solo practice" -- self-employed, practicing alone in a private office, charging a fee for each service, operating as a small business and making a substantial living.

Even today, that is in large part an accurate picture.

Nearly half of all active nonfederal physicians in patient care are still in solo practice, not counting hospital interns and residents, according to American Medical Association statistics.

And most of the rest are only a step removed from solo practice. They have partners or share an office with other doctors in business for themselves and bill patients using the traditional fee-for-service system.

Now this situation is about to change drastically.

In the next 15 to 20 years, doctors can expect these developments, numerous doctors, health economists and other experts predicted:

The proportion of doctors in solo practice using fee-for-service payment will plummet, perhaps becoming insignificant.

Just how much solo practice will decline is in dispute. Some authorities, like Gerard Anderson of the Johns Hopkins School of Public Health, believe that as few as 25 percent of doctors will be solo practitioners by the year 2000. Others believe the percent will be lower.

Half or more of the nation's doctors will be working for a salary or a salary-like fee-discounting agreement paid by health maintenance organizations (HMOs) and other medical groups.

This will occur because more and more patients will be treated under arrangements -- such as HMOs -- in which they receive whatever treatment needed for a flat, annual fee without added charges. These arrangements require the HMO to keep doctor costs fixed and predictable, as in a salary system or a contractual fee-discounting arrangement.

While the number of doctors working substantially full-time for prepaid group health plans (HMOs) is quite small today -- estimates range from 7,000 to 18,000 -- it will increase enormously as HMOs capture a larger share of the market.

HMOs have 18.9 million members now, up from fewer than 3 million 13 years ago. While this still represents only a small fraction of the U.S. population, some analysts believe HMOs will capture half the nation's patient population within another decade or so.

New types of "vertically integrated" healthcare systems will emerge as providers attempt to keep costs low. These large healthcare corporations -- some nonprofit and some for-profit -- simultaneously sell health insurance and operate HMOs, hospitals, nursing homes, medical labs and medical supply companies. They rely on salaried doctors or doctors working on a discount.

Today there are just a handful of these companies. But they, too, are expected to grow enormously.

A large portion of the doctors who aren't working for HMOs or health-care corporations will combine into group practices to economize by sharing expenses and to offer a wide range of "one-stop-shopping" services, making use of a lot of medical techniques and expensive machinery previously found mainly in hospitals.

As a result of all these changes, some authorities believe that by the end of the century, 50 percent or more of the nation's physicians will be working on a straight salary basis.

"We believe that by 1995, the majority of physicians will be organized into collective bargaining units and will be paid on a salaried basis," wrote Richard Averill, a vice president of Health Systems International, and Michael J. Kallison, a health law specialist in a recent issue of the magazine Review, published by the Federation of American Health Systems.

Others, like James D. Bentley, an official of the Association of American Medical Colleges, say perhaps only a third of all doctors will be salaried. But even that would be a huge increase in the proportion of salaried physicians.

Physicians' incomes will drop. "Physicians' incomes have gone down 1 percent a year in real after-inflation terms" for several years, said Robert J. Rubin, former assistant secretary of Health and Human Services, now in charge of policy analysis at ICF Inc., a research firm.

"That trend will continue and perhaps accelerate. They will still do well, but the days of astronomical incomes are going to change, particularly at the upper limit for some surgical specialties."

Papers presented at a Cornell University conference earlier this year came to the same conclusion: physician income is already declining and the earnings potential of medical specialties, particularly, is likely to diminish.

Fifteen years from now, the average doctor in patient care will net $90,000 a year after expenses, but before taxes, based on 1984 dollars, Anderson says. That compares with $108,400 today. Bentley said some analysts believe the drop will be greater -- to between $60,000 and $75,000.

While this is substantially less than now, Bentley said, it is still far more than the $26,433 median family income for the nation as a whole. "We won't have any physicians driving cabs," said Bentley. "I don't foresee that."

Of course, these numbers are "soft" and conjectural. But while there are disagreements and uncertainties about specific numbers, there is relatively widespread agreement on the overall trends, because of three major developments that are beginning to change the medical landscape.

One is the enormous pressure, generated over the last decade by government, business and patients, to hold down medical costs.

The second is a growing surplus of doctors, caused by heavy medical school enrollments over the past generation (now starting to decline) plus admission of foreign medical graduates.

Third, there is a surplus of hospital beds nationally.

Some experts, like Paul Ellwood, founder of Interstudy, a Minneapolis medical think-tank and now head of his own health economics firm, believe that this cost-cutting pressure will result in a substantial consolidation of health services and organizations over the next decades.

Huge organizations will emerge to handle virtually every aspect of health care. They will run health plans, they will hire their own doctors on salary or on discounted-fee arrangements, they will market insurance, and they will create HMOs. They may buy and operate hospitals.

Providing a full range of services, they can capture patients with simple, one-stop shopping appeals; they can channel their HMO patients into their own hospitals, making sure they don't have too many rooms empty; they can make deals with other hospitals to get low rates when they send them patients; they can hire young doctors cheap because of the surplus.

Already, such consolidation has begun. CIGNA Corp. (formerly Connecticut General Life Insurance), a leading health insurance company, already owns 18 HMOs, some dental HMOs and a share of one hospital. CIGNA continues to sell health insurance and has arrangements with groups of doctors and hospitals to provide it low-cost services, said executive vice president Robert Patricelli.

Hospital Corp. of America and Humana are moving in the same direction, and nonprofit hospitals and HMOs are expected to do the same eventually. In some cases, a hospital will expand and start selling health insurance and running an HMO and its own hospital supply company. In others an HMO will start creating a "vertical" business of this type, or a private doctor group.

By some time in the 1990s, anywhere from eight to 20 large vertically-integrated organizations will provide the bulk of medical care in the United States, Ellwood predicted.

Although there is wide agreement there will be consolidation, some say this prediction is too extreme. Anderson, for example, believes there will be substantial consolidation, but not nearly as extensive as Ellwood predicts, because, he said, "It isn't clear that such vertically integrated health systems can produce health care at substantially lower costs than free-standing hospitals and private doctors."

The basic model for all these developments is the HMO.

The fee-for-service system, in which the doctor sets the fee, and similar open-ended charging systems by hospitals and nursing homes, has been blamed by many for the rise of health costs.

The favored solution has been a move toward flat-rate payment systems, like those used by HMOs in which the hospital is given a single payment regardless of how many days the patient stays or how many tests he has. With a fixed premium, the HMO must operate efficiently or lose money.

Studies have shown that well-managed HMOs, such as the Kaiser-Permanente program, usually have fewer hospital days and lower doctor costs per patient than if the patient were being treated under the traditional fee-for-service system by a variety of doctors.

The one-time scarcity of doctors will soon turn to a surplus, according to the federal Graduate Medical Education National Advisory Committee (GMENAC) and the Department of Health and Human Services' bureau of health professions.

The number of physicans, now about 500,000, double what it was in 1960, will rise to 594,600 in 1990 and 706,500 by the year 2000, projects the BHP. This would mean an oversupply of about 35,000 in 1990 and 52,000 in 2000. The GMENAC study, using somewhat different assumptions and bases, put the oversupply at 137,200 in 2000.

The expected result is that young doctors will find it more difficult to get started in private practice and will turn increasingly to salaried jobs. They will also work for less than in the past.

Another reason why solo practice will be less attractive in the future is the large tuition loans many doctors must repay after medical school. A new medical school graduate can easily have a debt of $50,000. Then, said Donald Fisher, executive vice president of the American Group Practice Association, it can cost an added $80,000 to start an office, and take several years before a profitable practice is built.

Many young doctors prefer to go into HMOs or an established physician's practice and start earning a salary immediately. Others go into private practice but join one of several new types of discounting organizations in which they agree to provide services to patients referred by the organization for a discount from the normal office fee. The hope is to get more patients by giving a discount.

In one version, the preferred provider organization (PPO) agrees with a buyer of services -- for example, an industrial company -- to refer the company's employes to a member doctor at the agreed upon price. The doctor can also treat his or her own regular patients in the traditional way. Another version, the independent practice association (IPA), is similar but the purchaser of services is typically an HMO.

Although the doctor surplus has not yet hit, there are signs it is beginning. In some areas with many medical schools and lots of graduates, some doctors are starting at HMOs after three years of internal medicine residency and completion of board certification exams for salaries as low as $35,000.

Dr. Marc Bard, chief of doctor recruitment at Harvard Community Health Plan, said $50,000 was about the going rate for a good starting internist after three years of residency in Boston and Chicago and many other cities. But he said the downward pressure on pay will start in a few years when doctor surpluses start growing.

These projections rely on the assumption that predicted trends will take place. But Bentley warned that that sometimes just doesn't happen.

"What if some disease of epidemic proportions broke out?" he said. Economy and cost-saving would be set aside and people would pay doctors anything to save them. "We'd be spending money like you never saw, confounding all the predictions."