It's simply been too long. More than eight years after her classic comedy series went into reruns and after a spate of movies that left us with far more tears than laughs, the real Mary Tyler Moore is back at last, bringing with her a fine comic touch that rivals the best television has to offer.

For seven seasons she turned on Saturday night with a smile. Now CBS is counting on Moore to brighten Wednesday evening -- for both the viewer and the network's sagging Nielsen ratings.

For the network, Moore is the probable answer to the question, who can take a nothing Wednesday evening and suddenly make it all seem worthwhile? Plunked into the 8 p.m. timeslot, Moore gives instant strength to a CBS lineup that has featured the lame and the halt, the already-failed "Stir Crazy" and George Burns' comedy anthology, plus the earnest but ailing "Equalizer." Can she do for CBS on Wednesday what Bill Cosby did for NBC on Thursday?

For the viewer, this midseason replacement show is a chance to welcome back to series television one of its finest talents. If Lucille Ball is the queen of situation comedy, Mary Tyler Moore is the princess.

And if Moore's new show, titled simply "Mary," brings smiles to viewers and network executives alike, it also brings a lilt to Moore's voice when she talks about, essentially, coming home.

She first felt the urge to return while doing "Begin Again Finnegan" with Robert Preston for HBO.

"It was while doing that show I remembered how much I missed romantic comedy," she said, "and how good I am at it . . . After discussing it with my husband -- he's at Mount Sinai in New York, and it would mean commuting -- I called the president of our company."

Mary Tyler Moore Enterprises, the company formed around "The Mary Tyler Moore Show" some 15 years ago with Moore's former husband Grant Tinker as president, made her feel right at home. Moore is back in the same trailer she lived in and with many of the same crew members she worked with while doing "The Mary Tyler Moore Show."

The show, set in Chicago, is shot before a live audience in California, making Moore, as they say in show biz, bi-coastal. "But it's better to be commuting than to be at home all the time and moping around," she said.

After a series of well-publicized personal problems, Moore sounds genuinely enthusiastic about the show and about being back in the grind of turning out a well-crafted show each week. "I love it," she said of the regime. "But then, I'm part masochist."

And if Moore is returning to television after going through some changes in her personal life, she is also bringing in Mary Brenner a character that could well be Mary Richards of the "Mary Tyler Moore Show" with changes brought on by time. Richards was single, sweet and vulnerable eight years ago as she learned to cope with such icons of the real world as Mr. Grant ("You've got spunk, Mary. I hate spunk.").

Now we find the six-time Emmy winner playing a divorced Chicago woman working for a fashion magazine, still showing a bit of vulnerability and naivete, but with a bit of an edge too. Reminded that she's scheduled to attend a convention of sportwear salesmen, she snaps, "Uh-huh, 500 little bald men in tennis outfits whacking me on the fanny with their rackets."

When the magazine folds she's out on the Chicago streets looking for work. She splashes into the journalistic swamp of the Chicago Post, a mythical third-string Windy City tabloid (headline of the day as she goes in for an interview: "Arsonist Sets Self on Fire").

She also lands in the middle of the show's ensemble of wacko players, a group that's a lot closer to the edge than the gang back at WJM-TV.

James Farentino is the managing editor of the paper, for which Mary will write a consumer help-line column. "He's brilliant at light comedy," said Moore. Indeed, in the premiere episode this week, he shows a terrific sense of timing.

John Astin plays Ed LaSalle, a theater critic with an ego reminiscent of Ted Baxter's. "And he's on the make for everybody," added Moore.

David Byrd plays a character who, depending on how he's developed in the show, will either convulse or repulse viewers: He's Tully, a legally blind copy editor. "They can't fire me" is his constant refrain as he cites the union contract chapter and verse. "We have to be careful," Moore said, "not to have his part turn into nothing but a series of blind jokes."

Katey Sagal is Jo Tucker, Moore's deskmate, the writer of a cynical "Eye on Chicago" column who lives her life in a constant cloud. "She thinks smoking is the ultimate sport," said Moore. When it is suggested that she's bitter, Tucker says she'd prefer to be thought of as macho.

Back at Mary Brenner's apartment house (not destined to become the tourist attraction Richards' home in Minneapolis was), we meet her best friend Susan Wilcox, played by Carlene Watkins. She's a city planner whose personal life is out of control. The man she met yesterday and is having move in today is Lester Mintz, played by James Tolkan, well-heeled, with Vegas connections. What does he do for a living? If you have to ask, you don't want to know.

It's hard to believe that Moore has been in television for nearly 30 years. She was 20 years old in 1957 when she played Sam, the leggy, deep-voiced answering service operator in "Richard Diamond, Private Detective." Her face was never seen.

She established her comedy credentials during the five-year run of "The Dick Van Dyke Show," in which she played Van Dyke's wife, Laura Petrie, a former dancer who turned crying into an art form. Four years later sh was playing Mary Richards in a series that ran from 1970 to 1977 and helped make Saturday night on CBS must- viewing for comedy fans.

Watching "Mary" is a little like watching the moon emerge from an eclipse that's lasted for years. Her time away from situation comedy has corresponded to a period of her life that was anything but sunny. In 1978 her sister died of a drug overdose. In 1979 she and Tinker, now chairman of the board and chief executive officer at NBC, separated after 17 years of marriage and then were divorced. In 1980 her son killed himself with a shotgun.

Through it all, Moore's professional work was peppered with tear-jerker films. "First You Cry" was a movie recounting a woman's ordeal with breast cancer; "Ordinary People" earned Moore an Oscar nomination for her portrayal of a mother whose son nearly commits suicide, and in "Heartsounds" she played the wife of a man afflicted with heart disease.

"Those were the scripts that came across my desk," she said. "It was during that time that I started to miss comedy."

The personal ups and downs have continued in recent years, with Moore marrying New York cardiologist Robert Levine two years ago and spending time at the Betty Ford Center in California.

Now she's committed to a commuter marriage and the killing grind of a weekly series.

"It's exhausting," she said simply. "But I want so much for this to work." The basic chemistry for a successful show seems to be in place, but achieving a natural look on screen still takes effort.

"My selective recall of the last show was of . . . a cast that had such carefully blended talent," said Moore. "Now it takes some getting used to . . . I'd forgotten what angst is involved."

Moore's return to televison comes at a time when her kind of sit-com seems to be back in style.

"There's definitely been a resurgence of literate, sophisticated comedy," she said, mentioning "Cheers," "Kate and Allie," "The Cosby Show" and "The Golden Girls" as the best of the breed.

"If the three networks each go on the air with someone reading from the phone book, one of them will get a higher rating," she said. "But these shows show there's an audience that will opt for programs on a higher plane.

"And we're delighted to supply one."