Cheech & Chong, the most endearing and degenerate comedians in Hollywood history, enter their third feature, and outrageously amusing ramble called "Cheech & Chong's Nice Dreams," in a droll vehicular sight gag. A telephoto lens trained on the crest of a hilly stretch of highway reveals a car with the California license plate "BAD GIRLS" rolling into view, closely tailed by a second car identified as "BAD BOYS." Straggling behind is a grotesquely grinning, bobbing clown's head with a fat joint wedged between its teeth at a jaunty angle. This alarming apparition turns out to be not a funhouse totem on the loose but the roof ornament on an exhaust-spewing ice cream vending truck whose license plate announces "Los GUYS."

Los Guys are, of course, the habitually stoned, oblivious comic heroes, who appear to be sitting pretty as ice cream men whose open trade in ice cream bars fronts for a more profitable under-the-counter trade in marijuana bars. Business is evidently so good that the boys can afford to luxuriate in affluent daydreams, Cheech contemplating a holiday attended by obliging cuties and Chong the purchase of more electric guitars.

Opportunity seems to knock for Cheech when their truck is overtaken by a convertible carrying two voluptuous passengers, one of whom inquires, "Got any Big Sticks in there?" The saucy wenches speed away, and the heroes take up the chase, provoking 90 minutes or so of episodic but frequently hilarious diversion in the pursuit of hedonistic bliss.

Cheech & Chong have adapted their stoogey characters and satirical burlesque of the drug culture "life style" to the movies with remarkable ease and assurance. They seem tthe freshest and most imaginative comics to seize a creative hold on the medium since Woody Allen emerged more or less confidently in "BANANAS."

Their technique grew even smoother when Chong assumed directing responsibilities in their second outing, "Cheech & Chong's Next Movie." They may also prove the most satisfying two-man comedy team since Laurel & Hardy, in part because they've restored the idea of compatible foolish personalities. Their interplay is mercifully free of the abusive notes that spoiled some of the fun to be had in teams like Abbott & Costello, Martin & Lewis and even Crosby & Hope, where one partner usually played a sleazy sharpie to the other's ingratiating patsy.

Richard (Cheech) Marin and Tommy Chong make an incongruous match: The former is a short, mustachioed Mexican-American who grew up in Los Angeles and the latter a tall, bearded Canadian of Chinese and Scots-Irish parentage who grew up in Edmonton. Their temperaments and vocal styles are also humorously contrasted, with Marin's excitability and volubility finding a peculiarly effective sounding board in Chong's drawling imperturbability. However, their characters operate in befuddled intellectual harmony. Neither is smarter than the other. They drift around the sunny L.A. landscape in a mutually oblivious daze, surviving by the grace of dumb luck.

The more inspired brainstorms in "Nice Dreams" include a plush new domicile, a return engagement for Stacy Keach as the narc nemesis from "Up in Smoke," and a facetious bit for Dr. Timothy Leary. Cheech & Chong have moved out of their squalid East Los Angeles bungalow and into a beachfront mansion, supposedly to house-sit for the owner, a rock star on tour. The spacious outdoor swimming pool there conceals a subterranean laboratory. Beneath the aquamarine tarp a mad young chemist experiments with exotic blends of pot, while the boys help themselves to the surrounding crop. One gathers from his advanced state of abstraction that the chemist has already carried self-testing beyond the point of no return. So has the Keach character, Sgt. Stedenko. The boys' old adversary remains holed up in his office, so addicted to the weed that he's regressing into a different species.

Still, there's always been a double edge to the kind of stoned flipness perfected by Cheech & Chong. Unlike drug apologist, they've never pretended that habitual usage would improve character. At best, drugging is perceived as an irresistibly enjoyable vice and the jokes are underscored by the implication that prolonged indulgence will lead to permanent dopiness.

Leary's appearance as a crazed staff member at a private booby hatch called Casa del Whackos tends to confirm their outlook. You can't help recalling Chong's suggestion early in the film that it might be a good iead to start "a rest home for old hippies," since the gang isn't getting any younger. He thinks Laid-Back Manor would be a pleasant name for such a retreat.

Although they impersonate woozy druggerrs, Cheech & Chong make wide-awake movie comedies. The best sustained slapstick sequence in "Nice Dreams" is an intricately choreographed game of hide-and-seek provoked by Cheech's encounter wsith an old highschool flame, Donna (Evelyn Guererro), who gets it into her coked-up head that she'd like to party with both guys. The upshot is that Cheech ends up beating a naked retreat from balcony to balcony and down a glass-walled elevator while Chong scrambles for cover inside Donna's apartment, trying to elude her monstrous mate, a biker called Animal (Michael Masters) who makes an unexpected return.

The sequence reveals Chong's flair for visual comedy at its keenest. Cheech's dumbstruck takes throughout his nude ordeal are also worth savoring, although his funniest isolated bits occur elsewhere -- a brief consultation with the men in the audience on the question of taking advantage of a date who's just passed out and a poignant appeal for a scratch where it itches the most while confined briefly in a straitjacket at Casa del Whackos.

Although working from a farfetched pretext, Cheech & Chong develop authentic archetypes, characters who exaggerate the lazy, dissolute, promiscuous slob lurking somewhere in the back of about everyone's imagination. Even if you reject the behavior, you recognize the urge. Sometimes it doesn't pay to become too respectable, especially if your subject is the human comedy. You'll know Cheech & Chong are in trouble the day they also become over boys for People and Redbook.