"A Change of Seasons" will be remembered, and can be savored, only for its credit sequence in which Bo Derek of "10" returns to the screen bobbing up and down, topless, and in slow motion, in a hot tub. Up she goes, down she goes. Up she goes, down she goes.

Uppppp she goes. Downnnnnnn she goes.

And one can surely be forgiven for thinking, "Please God -- one more up-she-goes," particularly in view of the insipid and puerile romantic comedy that follows this crowd-pleasing curtain-raiser. The film, which opens today at area theaters, tried to make adultery as sickeningly cuddly as it was in "Cousin, Cousine," but it doesn't even have a convincingly phony kind of charm with which to pull the dubious mission off.

Once the game of bobbing for Bo is over, the picture, from a story by Erich "Love Story" Segal and producer Martin Ransohoff (who gave TV "The Beverly Hillbillies") settles down into a plot gimmick that was probably inevitable, considering the giggly age in which we live. When Anthony Hopkins, as a musty old duddy of a college prof, takes up with one of his students (Derek) and his wife, Shirley MacLaine, finds out, she stages a counter-affair with a granola carpenter (Michael Brandon) who's come to the house to install bookcases.

Trying to be adult about all this, the husband and his new mistress consent to sharing the couple's ski cabin with wifey and her boyfriend. The ingredients for a romp may be there, but screenwriters Segal, Ronni Kern and Fred Segal are more interested in exploring the dullest possible possibilities and in a 15-round bout of demonstrative soul-searching about the changing nature of marriage.

In effect, they're merely copping an elaborate plea for playing around. The characters as written (and as haplessly directed by Richard Lang), are paper cut-out dolls being trotted past wearying signs of the times. he's discovering what being a man is. She's discovering what being a woman is.And so on and yakety-yak.

Has there even in history (since Eden anyway) been such epidemic sexual self-consciousness? It's taking the fun out of everything. Even bedroom farce.

The writers try to throw in a few twists and turns, but they tend only to make foolish matters more so. When Mary Beth Hurt as the couple's college-age daughter shows up at the cabin and begins to sputter beguilingly about her parents' morals and lack of discretion, the picture comes momentarily to life. Maybe it should have been built around this conflict, but the screenwriters reflect too middle-aged a viewpoint to risk anything so self-effacing.

Most of the movie takes place at the ski resort, where the couples pair off for awhile, then sit around waiting for complications. The young woman's father arrives, for instance and, after lover-boy takes a powder, he finds himself smitten with MacLaine.Wrong trees are barked up hither and thither; the writers don't seem to have any notion of where the right ones are planted.

Instead, they're preoccupied with such contortions as making the male lover sympathetic by having him fill in details of his tortured nomadic past. Yes, it seems that he lost his parents to a 10-ton truck and his wife and baby to herbicides. Really. This disclosure could be the funniest scene in the picture, but it's meant to be ominously serious (Couldn't the parents have been victims of a nuclear accident? That would put the perfect crumble on the old cookie).

MacLaine is twinklingly sanctimonious, which is what she usually is when given too much tether, and Hopkins is a stammering, puffy-eyed medley of mannerisms. Derek is wonderful only until the actual picture starts and the credits are over. She does have another slightly nude scene, behind a shower door, with Hopkins ludicrously suggesting to her that they go off to a concert of Shostakovich and Bartok. Perhaps he's interested in her mind; in a party-pooping way, it would figure.