The Federal Communications Commission yesterday said two California-based "dial-a-porn" services appear to have violated rules requiring them to take steps to assure that minors do not place calls to them.

The FCC said in letters to the companies that they appeared to have violated FCC rules by failing to require the use of credit cards or access codes. The companies were given 30 days to respond.

The action marks a commission effort to crack down on companies under its own authority, FCC general counsel Diane Killory said. The commission has previously referred at least four cases to the Justice Department for action, she said.

The services make money by charging callers for listening to sexually explicit messages. A 1983 federal law, however, makes it a crime to allow minors access or to transmit the messages to adults who have not given their consent.

Rep. Thomas J. Bliley Jr. (R-Va.), sponsor of that law, welcomed the commission's action yesterday but said it had not come fast enough. "I've been disappointed that they haven't moved more aggressively," he said.

FCC Chairman Dennis Patrick said the commission's efforts have been hampered by litigation in federal court challenging its rules. He also noted that the FCC has had to conduct its own investigations after receiving complaints of violations.

Patrick promised that the commission would be "diligent" in enforcement. Other cases are under review, he said.

The FCC's first approach to enforcing the law was to enact rules limiting operation of the services to between 9 p.m. and 8 a.m. Eastern time. However, those rules were overturned by the U.S. Court of Appeals for the 2nd Circuit in New York in November 1984.

The commission then shifted gears and in October 1985 issued an order requiring operators to use special access codes that would be assigned to users in advance or to require credit card numbers. Intensive litigation concerning the rules followed, helping delay their enforcement.

The two actions yesterday grew out of letters to the FCC from parents alleging that children in their homes had freely called the sexual phone services offered by Audio Enterprises Inc. of Mill Valley, Calif., and Intercambio Inc. of San Jose, Calif. The companies could not be reached for comment.

The FCC stressed that yesterday's letters do not constitute a finding of wrongdoing by the companies. Under the law, the commission can fine operators up to $50,000 per day. The Justice Department can seek injunctions, fines or sentences of up to six months in prison.