It sounds implausible when you regularly hear that, unless you have fatal symptoms, the next available doctor's appointment is weeks away. But the forecast now common around the doctoring business is that the M.D., America's last fully guaranteed ticket to personal affluence, is en route to an economic nose dive.

"I think the crisis will come with fair rapidity for many doctors", says a medical management consultant, ThomasE. Zirkle, in the newspaper of the American Medical Association. "Physicians who are doing well at present", he added, "should anticipate the problemand prepare to meet it in two to three years".

While concerned physicians batten down the hatches, the rest of us should refrain from expressions of sympathy andcontemplate the prospect of trends, healthy and otherwise, that are beginning to show up in the economics of health care. Relative to other means of livelihood, traditional medicine will remain an extremely rewarding profession, mainly because the American people crave its attention and demand that the political system provides ways to pay for it. This it does through Medicarefor the elderly, Medicaid for the poor, tax-free health insurance policies for employed and massive federal assistance to medical education.

The result is that health care is thebiggest industry on the American landscape -- now running at the rate of about $220 billion a year. But all that wealth has inevitably attracted many competitors. And that's what the longtime kingpins of the systems, the M.D.s, are beginning to feel.

Some of the strongest pressure comes from the nation's expanded network of medical schools, which are now producingdoctors at a rate that far exceeds population growth. With over 16,000 new M.D.s coming out each year, the federal government estimates that the number of practitioners per 10,000 of population will rise from the 1950 levelof 14.2 to nearly 25 by the end of the 1980s.

The economic significance of these swelling numbers is just beginning to be reported around the medical system. While horror stories from medically underserved ghetto and rural regions attract a lot of press attention, what's gone largely unnoted is that in most geographically and culturally attractive parts of the country, the steady refrain in, "We don't need any more doctors here."

In some of these areas, there are reports of real competition among physicians trying to attract or hold on to patients: price cutting, extended hours, convenience of location -- all ofthese are beginning to show up in some of the country's more doctor-laden regions. In the booming Sun Belt areas, for example, the newest medical trend istoward low-cost, privately owned, walk-in medical clinics in shopping centers.

The other side of the coin finds doctors; like other threatened economic interests, fighting to hang on to what they've got, and often being tough about it. Zirkle, for example, tells of "resort-area group practice which is very concerned about the possible influxof additional physicians. This group, like many others, is trying to tie up the whole district for 50 miles around by establishing satellite clinics. The idea is to protect census from future erosion and to provide a pipeline for referral to their main facility. They want to make it impossible for a new docor to come onto their turf exept as a member of their group. I don't think this could have happened 10 years ago".

While doctors mobilize for turf wars,it may be that the most interested onlookers are situated in what's only lately coming to be recognized as the true power center of American health care -- the health insurance, or reimbursement, system.

Medical costs have zoomed without restraint for the simple reason that the insurance systems, public and private, have had no incentive to restrain them. Under the doctor-written, self-serving rule that bills are okay if they are in amounts that are "usual, customary and reasonable," the insurers have routinely paid up and then passed the costs along to the public through higher premiums and taxes. In some instances, when a foolhardy insurer has insisted on trying to hold down costs, physicians have responded with boycotts of the insurer.

Those tactics have drawn the legal ire of the Federal Trade Commission and, in several important cases, the offenders have agreed to back down. Meanwhile, with health insurance premiums continuing to rise in a static economy, there is an outburst of interest in how to do it cheaper. Pre-paid plans -- health-maintenance organizations -- are getting a second look from industrial firms that pay health insurance premiums as a fringe benefit. The insurers are talking tougher to the providers of health care, and patients -- who still must bear some costs personally --are showing a new interest in the price of care.

Add to that the army of young physicians now coming into practice and it's plain that, for many of its practitioners, doctoring is about to emerge from a golden phase and pass into a period of unusual economic turbulence.