The American Medical Association must let doctors advertise their fees and compete for patients, the Federal Trade Commission ruled yesterday.

In a final ruling on a December 1975 FTC complaint that the AMA increased doctors' incomes by squelching price competition, the FTC ordered the group to tell its 200,000 members they can do almost what they please to solicit patients.

But the commission said the giant medical organization may set and enforce "reasonable ethical guidelines" to prevent "deceptive" or unsubstantiated" claims.

That part of the decision overruled the 1978 recommendation of Ftc Administrative Law Judge Ernest Barnes, who would have barred any AMA regulation of ads for two years, and allowed AMA guidelines then only with FTC approval.

AMA officials in Chicago said they were pleased with that part of the decision, since it "endorsed the position" they had taken through the case. But counsel Newton Minow said the AMA will appeal any clauses that it feels will still prevent medical societies from acting against "deceptive or other unethical practices that may harm or mislead patients."

Since the filing of the FTC complaint in 1975, and particularly since a 1977 Supreme Court decision striking down the American Bar Association's restrictions on advertising by lawyers, there has been little effort by the AMA to enforce its rules against advertising.

Nationally, however, few doctors have been advertising, though California newspapers have been sprinkled with ads from cosmetic surgeons.

Minow said he fears deceptive advertising may increase, given the fact that only 53 percent of the country's 400,000 doctors (though 72 percent of its office-based doctors) belong to the AMA.

He cited AMA and some patients' testimony before the FTC saying some people have been lured to doctors, especially California cosmetic surgeons, who promised to improve their looks, and then harmed them.

Many patients, said Minow, "begged to be protected from [advertising] quacks who butchered and mutilated them."

Dr. Charles Epps, District of Columbia Medical Society president, said he, too, fears more doctors may start making "self-aggrandizing statements and putting out neon signs saying, 'I'm the best gall bladder surgeon.'"

So far, he said, all D.C. Medical Society members have complied with rules permitting doctors to list or advertise their names, qualifications and even fees, so long as they don't mislead patients.

The AMA consists of many state and local member groups, like the D.C. society. It is the local groups that exercise the real restraints.

The FTC dealt with this by ordering the AMA to disaffiliate itself for a year from any state or local society disobeying the FTC rules.

And it called any current rules that permit some fee advertising, a such as the District's, "limited" and "ambiguous," and said its order is needed to prevent a renewal of the most restrictive practices.

"It is especially important that price advertising remain as unfettered as possible," said the opinion written by Commissioner David A. Clanton. He said past AMA actions have "severely" inhibited health care competition and caused the public "substantial injury."

The commission also ordered a halt to any lingering discrimination against doctors who choose to work for prepaid medical groups such as health maintenance organizations (HMOs) that have been growing in number -- or other employers. The AMA and its members societies long fought bitterly against such "corporate medicine," as they called it, and sometimes expelled members who took salaried jobs.

The FTC said the AMA may make regulations, however, against uninvited solicitation of individuals -- presumably the aged or mentally infirm -- who are particularly "vulnearable to outside influence."

The FTC case is part of a series of rulings that have given lawyers, engineers, druggists and optometrists the right to advertise freely.

One nationwide discount chain is said to be thinking about opening medical centers in its stores, such as the optometry and dentistry centers that already are operating.