The Federal Trade Commission has begun an investigation to determine whether airport authorities all over the country are violating the antitrust laws in the process of contracting out space to auto rental firms, sources said yesterday.

The investigation has not been made public.

Directed by the FTC, the staff has sent letters to all the major auto rental firms seeking information on their dealings with 59 major American airports, including the three in the metropolitan Washington area - Washington National International and Baltimore-Washington International.

The companies have been asked to send the commission copies of their current contracts with the named airports and any other documents they have concerning the car rental rates they are to charge and the advertising they may engage in, as well as any information about their need to obtain prior approval before the company can make any changes in the auto rental rates at airport locations.

The FTC is said to be especially interested in two apparently common provisions of the contracts used by airport authorities. One prohibits the car rental company from changing its rates without the airport authority's permission. Another requires the company to charge the same rates that are being charged at other locations in the same area. especially at nearby airports.

Sources said contract provisions may well be subject to challenge under the antitrust laws on the grounds that they may restrain trade and limit price competition, even though the practices are sactioned by local, state, or federal agencies.

Expect for Washington National and Dulles, which are owned by the Federal government, all the other airports are state or municipally owned.

Although actions taken by the state bodies were generally presumed to have an exemption from prosecution for violations of the federal antitrust laws under a doctrine enunciated by the Supreme Court in 1943, the general trend in the court in the past few years has been to break down the use of "state action" as a defense in antitrust cases. In three recent cases, the court has delared illegal activities undertaken by state-sanctioned groups - like a state bar - when challenged as price-fixing or other violations of the antititrust laws.

The court has not declared its earlier enunciated doctrine invalid but has looked closely at the activity being challenged to see if it is an essential state function with a valid state interest, and has found several that did not warrant an exemption.

Should the commission decede after the probe that the airport authorities are engaged in practices which may violate the antitrust laws or the FTC Act, but that it doesn't have the authority to challenged them, it could issue a public report or send recommendations for legislative change to Congress.

Airport official contracted yesterday contend the contract provisions are a way of "making sure the public is not being gouged.

"It's been this way 25-30 years," one official said. "These concessions have a captive audience; we don't try to determine waht they charge, but we do try to keep things reasonably equal, and make sure they won't raise their rates after they get their lease."

Last year, the agency accepted consent orders from the nation's three largest auto rental companies - Avis, Hertz and National - prohibiting them from attempting to monopolize the car rental business, The orders settled a 1975 complaint charging the big three with conspiring to keep their smaller, lower-priced competitors out of the convenient airport locations which greet air travelers.

The Supreme Court yesterday refused to temporarily block the Federal Trade Commission from giving Congress data on uranium reserves, which the Exxon Corp. and two other companies claim would disclose their trade secrets.

Two lower federal courts had rejected the companies' interim plea to block the disclosure, and a U.S. District Court here had dismissed the entire suit.

The U.S. Appeals Court still has before it a company request for a 10 day notice before the commission turns over any material.