One of the slightly wonderful things about TV is its capacity to juxtapose the sublime and the ridiculous without moment's notice or trace of shame. One instant you're near tears over an emotional cliffhanger about a stricken fellow human and the next you are face-to-face with an ape beating up on a suitcase.

Tonight on television a viewer can watch one of the great ballet dancers of the century do a hoof de deux with a pink pig in a tutu, experience the sobering and reverberant trauma of a modern American divorce, and watch in helpless bemused horror as two literary titans slip-slide around in a fithy mud pile of their own making.

It's persistently fascinating the way this fabulous neutral space, the electronic canvas of the television screen, can host sequentially ennobling and degrading images of people and the universe. The beauty of it is, you seldom know exactly what the nature of the next image will be. This is one of the reasons why the contention that TV fries and dries the brain is fundementally insupportable; on TV, surprise springs enternal.

Pity the house without a television set on - a night like tonight.

Sometimes the sublime and the ridiculous are virtually indistinguishable. The idea of ballet superstar Rudolph Nureyev, pillar of grace, appearing as the guest on the silly, syndicated "Muppet Show" is both tantalizing and disquieting, but the result, on Channel 7 at 7:30 tonight, is unrelievedly magical. It's a warm and uproaring triumph for Muppets and pillar alike.

Certainly there's no assault on Nureyev's dignity to equal the Ken Russell movie disaster "Valentino," in which Nureyev recently starred. He may be considered past his prime by hardened balletomanes but, if so, his "Muppet" turn suggests ballet's loss may mean the emergence of a pretty grand entertainer for the world at large.

Nureyev is also, it becomes quickly clear, the best of sports.

His numbers on the show, taped in London, include a steambath duet - Rudi in white towel - with the ever-predatory Miss Piggy, to "Baby. It's Cold Outside"; an excerpt from the ballet "Swine Lake" danced by Nureyev and a huge ham, literally, of a ballerina; and an affectionate reprise of the title tune from "Top Hat" sung and danced a la Astaire.

It's doubtful there will be anything as purely and blatantly hilarious as the "Swine Lake" routine on TV for some time to come. The part of the pig is played, through clever videotype editing, both by dancer Graham Fletcher and by a great fat dummy.

Nureyev's singing may be bottom-drawer but when you have this much charm, you can get away with murder, at least of language. It's especially endearing that instead of singing "Oh, I'm putting on my top hat," he sings, in formal English doused in Russian dressing, "Oh, I am, pooting on my tope hat."

Jim Henson's Muppets poot on tope hat as well for this occasion, one of the merriest ever for a repeatly fresh and funny series - a series, we may remember, that all three networks rejected two years ago when it was offered them. "I'm having a great time," Nureyev says midway through tonight's show, and it's obvious he means it.

"On a beautifully day this, I want everything to be nice," says mom at the picnic. "But it isn't," says her daughter. Too true. A few minutes later the woman's husband tells her he's leaving her because there isn't enough "joy" in his life and "I can't function without joy."

ABC's "Breaking Up," a two-hour film at 9 o'clock on Channnel 7, may have superficial resemblances to other tales of domestic schism, but Loring Mandel's script, Delbert ("Marty") Mann's direction and the lead performances by Lee Remick lift it well above the standard.

Though not essentially a problem drama about divorce in America, the Mandel script does seem shrewdly attuned to the new threats to marriage that come from living in the selfindulgent Me-days of the '70s. The notion that one should always put oneself first is incompatible with the concept of marriage and family, and it also adds, in this case anyway, a distasteful new twist to the perennial middle-aged male's urge to rove.

Though much of the film details the wife's victimization - propositions from old friends, seductions from potential employers, accusations from her own kids - the character finds the strength to construct a new life on her own, and the final moments show her bruised but victorious.

Whether this denouement represents a cathartic feminist conquest or simply one giant leap for self-reliance is a matter of personal interpretation. Perhaps as many men as women will offer a silent cheer, however, when the wife finally tells her born-to-be-free ex-husband, just after he attempts a mea culpa crawlback, "You can't keep your options open for the rest of your life."

Granville Van Dusen is basically blank as the husband, but Remick is solid, sensitive and unerringly believable as the wife. The script structure may be episodic and moment-by-moment, but Remick makes these moments add up. Theyaren't played for melodramatic juiciness, and yet such scenes as one in which, awakened from a nightmare, she turns to the husband who's no longer there, have tremendously truthful impact.

"Breaking Up" is going to hit home in millions of American homes.

Such slop Such slime. How sordid.

What fun. What laughs. How delightful.

Dick Cavett calls his back-to-back chats with former sparring partners Gore Vidal and Norman Mailer "a good, old-fashioned literary free-for-all" in his introduction to tonight's special one-hour "Dick Cavett Show," at 11 o'clock on Channel 26.

But he's just trying to put a proper public television tuxedo on a disgustingly entertaining Bowery bum. Literary? When the literati heap gunk on one another, that is not a literary event. Not since Jacqueline Susann dined alone has there been a less literary event than Norman Mailer's tiny-fisted attack on Gore Vidal at an October party in mad Manhattan.

There's nothing literary about Cavett's clumsily belated rematch, either, but as frivolous and nasty confrontation television, it's surefire trash, and this despite the fact that Vidal and Mailer do not appear together but instead are each given a separately-taped half-hour.

Hence there's nothing quite so high-charged and electrifying as the first TV Vidal-Mailer encounter, a fondly remembered donnybrook on Cavett's old ABC late-night show in 1971. Cavett plays an excerpt tonight. It was one of his finer performances; he took something of a stand (against the pugnacious but amusing Mailer) and behaved something like a person. So, naturally, he apologizes for this display tonight, telling Vidal, "I seem to have blown my cool." Big deal.

Vidal's on first, and he says of Mailer, "he's been finished for some time as a writer," he's full of meaningless remarks," "his writing is perfectly terrible and has been for some time," and, perhaps choicest of all, "he reads nothing at all - he's never read me . . ."

He also refers to Mailer as "that monkey" and as a "fat little hen."

Mailer, later, says of Vidal, "he's such a bad man, he's so evil, he's such a liar, he's really such a corrupt and unpleasant piece of work when you get to know him" and calls Vidal "a little demented" and "a vicious liar." Great stuff.

Unfortunately, Cavett tries to smear a little redeeming social value over this childish imbroglio and thereby threatens to take the life out of it. It's also annoying that he is solicitous with Vidal but argumentative with Mailer ("Oh now, Norman," he complains at one point). Partly as a result of that, and partly because Mailer's much more of a verbal virtuoso than Vidal, he easily wins the contest, however deplorable the itsy-bitsy violence that set this all in motion may be.

Cavett's obsessed, too, with reminding us that both of these gentleman authors are his "friends" and says that "nothing would please me more" than for them to become pals again. Who cares what would please him, more or less?

Cavett's show has been improving in recent weeks - his cutey-pie posing has been generally under control - but he continues to display an appalling talent for missing the point and lacking an attitude.He actually behaves on the Vidal-Mailer show as if something important were at stake in all this. Why doesn't he just relax and let us all be handsomely diverted by a scrumptious vicious brawl?

He had a little assistance in taking some of the fun out of the program; lawyers went over it in their mean-spirited way and removed an insult or two, mainly Mailer's apparently, and chiefly remarks not directed at either of the participants. PBS advisers, for instance, that the phrase "sack of pus," as used by Mailer in describing a third party, has been deleted.

It's still a swell show.