Johnny Carson has held discussions with NBC-TV about his contract expires, possibly as early as Oct.1.

According to a source connected with the late-night show. Carson wants to leave Oct.1, the anniversary of his 17th year as host, because of a dispute with NBC President Fred Silverman over the performer's many nights off. Carson is under contract with NBC through the spring of 1981.

Silverman, who left top-rated ABC to shore up NBC's faltering ratings, has demanded that Carson increase the number of his appearances on the program, the source, who asked not to be identified, told Associated Press.

Carson told UPI in Los Angeles yesterday, "I am having discussions with the network about leaving the show before my contract expires in 1981. But I have not set any deadlines or issued any threats about when I would leave."

A network statement said, "NBC has a contract with Johnny Carson that continues to the spring of 1981 and we expect him to honor it."

Carson said he initiated the disscussions with Silverman but has not spoken to Silverman since March 18.

I decided to leave because I don't think I can keep up the level of quality in the show anymore," Carson said.

"I am no longer able to bring to it everything I would like to do.After 17 years I am mentally and emotionally tired.

"I don't want to go the full two years and I've expressed a desire to leave. There is no battle, but the network wants me to stay. I am not threatening to leave and haven't set a date."

"I realize my timing probably isn't very good because there are programming problems in other areas at NBC. That's like saying the Titanic had a small leak. But there are no fights and no threats."

A television executive close to the situation said rumors of Carson's asking to leave the show began at CBS. The spokesman said Carson was interviewed by Mike Wallace for the "60 Minutes" program and announced on that show, which hasn't yet aired, that he would be leaving the "Tonight Show" earlier than the contracted date.

According to the insider, "Carson thinks NBC will let him out of the contract. But they won't. It will be a battle because many millions of dollars are involved for the network with the show."

Under his contract, Carson is required to appear on the "Tonight Show" three times a week 25 weeks a year, and four times a week 12 weeks a year-with 15 weeks' vacation.

This year, for the first time in his 17-year run, Carson, 53, was facing a serious ratings threat from the other networks. Both ABC and CBS had failed to defeat Carson with talk shows of their own-ABC with Dick Cavett and Joey Bishop, CBS with Merv Griffin. But recent years showed steady eroding ratings for Carson and gains by CBS and ABC with reruns of old network cop shows.

CBS often beat the "Tonight Show" in late night ratings. The program always suffered lower ratings when Carson was on one of his innumerablenights off. Last month in New York, Silverman said he would encourage Carson to expand his presence on the program to four nights a week. "The man is a professional," Silverman said. then. "When he sees these ratings it'll kill him."

Silverman expressed confidence that he could talk Carson into a heavier work load, but stories about Silverman's efforts reached Carson before Silverman did and reportedly sparked resentment from the comedian. Carson occasionally made jokes about Silverman on the program, announcing, after Silverman canceled the entire fall lineup of new shows, that NBC now stood for "Nine Bombs Canceled."

More recently, Carson said that if the heavily promoted new series "Supertrain" folded, "NBC is selling the train to Amtrak, and Fred Silverman will be the conductor."

Carson is TV' highest-paid performer, with a reported annual income of about $4 million from the the show, endorsements and other performances. He took over as host of the "Tonight Show" on Oct. 1, 1962, succeeding Jack Paar, who was host for five years. CAPTION: Picture, Johnny Carson