For a fascinating contrast in changing sexual folklore, it would be tough to beat a double-bill of "Endless Love" and "Splendor in the Grass." Opening today at area theaters, "Endless Love" is a feckless new romantic melodrama about the dire consequences of runaway teen-age passion. The setting is urban and contemporary. Elia Kazan's "Splendor in the Grass," released in 1961, lamented the consequences of a harshly repressed teen-age passion, of high school sweethearts in a small midwestern town in the 1920s.

"Endless Love," directed with picturesque confusion by Franco Zeffirelli from an overloaded novel by Scott Spencer, seems to be "Splendor in the Grass" stood on its head. While socially revealing, this flipflop, which finds sexual abandon doing the sort of damage to Young Love that repression used to, wouldn't necessarily expose Zeffirelli's film to ridicule. After all, "Splendor" flourished commercially on the appeal of Warren Beatty, Natalie Wood and Zohra Lampert and a vividly evoked period setting.

But "Endless Love" appears destined to provoke laughter because Zeffirelli can't depict Spencer's precocious, overpowering love affair persuasively. The casting of the young lovers, upper-middle-class Chicago high school students named David Axelrod and Jade Butterfield, can be considered a textbook disaster. Spencer's overwrought fable depends on a convincing illusion of intense erotic attraction between characters too young to cope with it. Permitted to consummate their affair like nobody's business, David and Jade exacerbate family and generational conflicts, and that turmoil leads David to a mad criminal act that ruins several lives, including his own.

The sex scenes in "Endless Love" were an unfailing source of humor to a preview audience at the Avalon. The spectacle of newcomer Martin Hewitt, cast as 17-year-old David, and that dreamily old-timer Brooke Shields, the inevitable dumb choice for 15-year-old Jade, pretending to be in the grip of sexual ecstasy kept striking people as some kind of freakish sight gag. The most convulsive single response was provoked by a bewildering image of Hewitt grimacing so painfully in mid-copulation that people couldn't resist making obscene sport of his discomfort. Presicely what did Zeffirelli imagine the poor boy had gotten himself into? A wine press?

David Watkin's sumptuous lighting bathes Hewitt and Shields (and Shields' trusty "body double," who takes over the nude inserts) in savory tints of orange and blue, but the dramatic miscalculations are too profound to be finessed by lush color schemes. The book's sex is raw, but the rawness is integral to the idea of a passion inspiring heedless, self-destructive behavior. Zeffirelli was obliged to cut a good deal of naked fondling to avoid an X, and the sex scenes in the final cut look demure enough to qualify for a PG.

Zeffirelli and Judith Rascoe (who adapted Spencer's novel) start on firm ground, depicting David and Jade in the process of falling in love, a process described retrospectively in the novel. The books begins with David's criminal outburst, now a climactic sequence in the film (and shot very effectively), then goes on to deal with the long aftermath of his folly. Although the filmmakers are probably obliged to backtrack in this way, viewers familiar with the book are likely to feel even more of a sag after the climactic episode, because they realize how much ground remains to be covered.

David evolved into a insufferable literary sufferer, but there are big empty spaces left in the movie by the failure to suggest something of the involved mental life that fills the novel. Denied direct access to David's tormented thoughts and rationalizations, a movie audience must be content with Hewitt's opaque pantomime, which suggests nothing so much as an overeager novice who mistakes tics picked up from Montgomery Clift and James Dean for Deep Feeling.

The film is also caught in a slight time warp.The events in the book took place over a decade beginning in 1967. The affair derived a crucial originality from the idea that it belonged to the countercultural excesses of the late '60s and early '70s. Ironically, the movie audience responds with consistent approval to the humorous depiction of Jade's absurdly permissive parents, played by Don Murray and Shirley Knight. The Butterfields are so permissive that they can't honestly impose discipline on their kids when it's absolutely necessary. By the time they decide to cool off Jade's overheated romance, David has virtually taken up residence in her bedroom. Deprived of this outrageously liberal privilege, he takes violent revenge.

Moviegoers, seem to recognize the folly of Jade's parents and this element comes closest to evoking the period authenticity of the novel. While blurring the period, the movie also neglects to clarify social tensions that made the novel more suggestive. For example, David is the only son of a Jewish couple (embodied, inexplicably, by Richard Kiley and Beatrice Straight) who met while active in the Communist Party. There's a special sort of rejection in David's attraction to the free-easy Wasp Butterfields, so secure that they can afford to toy with the delusion that everything is permissible.

"Endless Love" solicits pathos with a case history so exceptional that it borders on the supernatural. The notion of transcendent teen-age sex would always defy credible depiction.

Zeffirelli seems more in tune with the adult characters, although he allows Shirley Knight to come on like a four-alarm fire when she's trying to seduce her daughter's boyfriend. The best youthful performer is Jimmy Spader, cast as Jade's brother Keith. Indeed, his technique is so sharp that it makes one even more conscious of the inadequacies of the leads.

Incidentially, isn't it past time to stop dangling Brooke Shields as erotic bait in movies where it's obvious that she doesn't comprehend sexality and everyone knows she's always doubled in sexually graphic interludes anyway? There's one weirdly funny take that seems to satirize this pretty string bean's excruciating lack of sexual consciousness. Tilting her head to one side and smiling like a simp, she looks amazingly like the friendliest extraterrestrial in "Close Encounters of the Third Kind."