NEW YORK (AP) _ There were first-day jitters, but by the end of CBS News' first ''CBS This Morning'' on Monday, the program started settling into a comfortable groove, thanks largely to newcomer Harry Smith's instant affinity for the format.

''It seems like we've been doing this for years,'' the former Dallas-based correspondent said at one point, and could have been talking about himself.

Co-anchor Kathleen Sullivan, CBS' highly paid, prize recruit from ABC, has been doing this for years, filling in on ABC's ''Good Morning America.'' But she seemed ill at ease, fluffing segues and clamming up when she should have been chatting. By the end of the show, though, she seemed to have relaxed a bit.

Sports correspondent Jim Lampley didn't make it into the studio in time for his first-hour report. Otherwise, the glitches were mostly technical. The awkward seating arrangement, with the anchors in the center of a U-shaped sofa, seemed to confound the camera operators.

In general, the show made good on producer David Corvo's promise not to try to anything rash, just offer an alternative to the other networks' morning programs. The result, at least in the first show, was a soothing style reminiscent of ''CBS Morning News'' when Charles Kuralt was anchor.

Segments concentrated on hard news. Smith interviewed Nicole Smigiel, the college student whose baby, given up for adoption, was found in the home of a New York couple now accused of child abuse in the death of another adopted child. Sullivan and Smith quizzed Jesse Jackson via satellite from Chicago where he is taking part in the political jockeying in the wake of Mayor Harold Washington's death.

The network had wisely retained weatherman-funnyman Mark McEwen from ''The Morning Program,'' and he seemed to fit in with his new colleagues easily. Dr. Robert Arnot also remains as health correspondent.

Robert Krulwich, another regular who will specialize in offbeat business and economics reports, used a large prop ''Mc'' to illustrate legal efforts by the McDonalds hamburger chain to keep others from using the prefix in front of business names, like Quality Inn's enterprise, ''McSleep.'' Sullivan promised his next report would examine the economic prognistication values of cooking grease.

Charles Osgood and Faith Daniels took turns doing newsbreaks. Osgood's reportorial talents were put to use on another humorous piece, an ode to a deluxe dentist, in rhyme.

The news division's morale was devastated last year when network executives canceled the ''CBS Morning News'' in the 7:30-9 a.m. EST time period and replaced it with ''The Morning Program,'' an entertainment-orien ted show with hosts Mariette Hartley and Rolland Smith in front of a studio audience.

''The Morning Program,'' a symbol of CBS cutbacks in news, failed to make any headway in the ratings, and CBS announced last summer that it was returning the time period to the elated news division.

CBS News announced in September that it had hired Sullivan to be co-anchor of the new show, now in the 7-9 a.m. EST time slot. CBS held tryouts for potential male co-anchors and offered the job to Canadian broadcaster Peter Mansbridge. But Mansbridge opted to stay in Canada and become the top news anchor for the Canadian Broadcasting Corp.

Harry Smith, a former Denver news anchor and talk-show host, had been with CBS since 1986.