After 10 months of secret negotiations it now appears that Johnny Carson and NBC are near an agreement under which he will remain as host of "The Tonight Show," with the program shortened from its present 90 minutes to an hour.

"Carson has responded to the idea that he only do an hour each night from 11:30 to 12:30," said a reliable source close to NBC negotiators. "There's no starting date yet as to when he would go to an hour because negotiations haven't been finalized. It involved money, too, obviously," the source said.

Brandon Tartikoff, president of NBC's entertainment division, said that the network had no official comment on his "informal discussions" on-going with Carson, but he admitted that the one-hour plan "is one of nine or 10 different things being talked about. It's one way of going," he added.

Asked whether NBC was getting closer to an agreement that would keep Carson on "The Tonight Show," Tartikoff quoted NBC President Fred Silverman as saying, "It looks promising."

Both Silverman and Tartikoff have denied a story published Monday in a Hollywood trade paper that quoted Silverman as saying he expected Carson to stay on the show beyond his April 1981, contract expiration period.

Sources close to Carson said yesterday that the mere fact that the two sides are discussing the one-hour show means that Carson most likely will stay.

"The one-hour idea is not new, it's what Johnny has wanted all along," said a longtime Carson insider. "There's never been a discussion regarding salary, the discussion has been time," the Carson associate said.

By all accounts, NBC has been scrambling to keep Carson ever since a contract dispute arose last year when Carson claimed that a California labor code section limiting personal service contracts to seven years meant that his contract actually expired last April.

"The odds of Carson winning his case (which is now in the hands of a court-appointed referee) were very slim because his contract has been renegotiated several times," said a source who claimed to have discussed the matter with Carson's own attorneys. "He really didn't have any grounds.It was that cut and dried."

On the other hand, "It is absolutely paramount to NBC to keep Carson, not only because of the money generated by "The Tonight Show" but also because they've had no luck in replacing him," the source said. "The people that they were specifically trying out as replacements, David Letterman and Richard Dawson, both had disappointing ratings."

According to another source, one of the options discussed with NBC concerns "a plan to do a 15-minute news show (opposite an ABC news show that is slated to begin in late March at 11:30) which would be followed by the Carson show from 11:45 to 12:45 a.m."

However, Tartikoff said he knew of no plans to develop an 11:30 news segment.

Regarding possible late night competition for ABC News, NBC News President William Small said late last night, "We haven't gotten that far yet -- no dummy programs have been produced or anything like that at this time."