Now it's time to really get stupid. If you think the fall season's been bad so far, consider the fact that you've already seen the cream of the crop.

Tonight begins the networks' real long months' journey into blight with three exercises in stupidity, although in the case of Mel Brooks' "The Nutt House," it's calculated stupidity, and in the case of another, "The Young Riders," we're not quite sure.

Mel Brooks' career has taken a downward spiral from genius ("The Producers") to a cut above average ("Young Frankenstein") to harmlessness if not humorlessness ("Spaceballs"). "Nutt House" follows in the footsteps of "Spaceballs" even with the addition of fellow zany guy Alan Spencer of the cult show "Sledge Hammer."

In order for this goofball genre to work, as in "Airplane" or "The Naked Gun," sight gags have to be piled upon witticisms upon outrageousness at such a dizzying speed that the viewer never has time to pay any mind to the mindlessness.

Tonight's hour premiere gives the viewer ample opportunity. "Nutt House," a parody of hotel films (grand or otherwise), isn't without humor. Some of the sight gags are terrific, such as a woman stuck inside a soundproof phone booth, unable to get anyone's attention. Cloris Leachman as the "housekeeper from hell" with padded bottom and breasts is often a riot, although she is merely grotesque in her other role, the hotel owner. Harvey Korman as the hotel manager isn't bad, either, although he's no match for Leachman's crypto-Nazi housekeeper.

But for every clever witticism there's a "Stop whistling, Dixie." Mark Blankfield as the near-blind elevator operator who can never find the right floor is funny once or twice, not a half-dozen times.

Even that would be tolerable if there weren't so many dead periods where nothing remotely funny is said or done despite the grating laugh track. Maybe that can be corrected when the show is reduced to a half-hour in its regular time period, but Brooks and Spencer are going to have to make "The Nutt House" nuttier. Fortunately for them, they have the "Night Court" lead-in that delivers tens of millions of viewers who are probably not in the most demanding mood.

* * THE YOUNG RIDERS 8:30 P.M. TONIGHT, CHANNEL 5 THEN 9 P.M. THURSDAYS

It may be that the folks most disappointed by ABC's "War and Remembrance" upset of CBS' "Lonesome Dove" for the best miniseries Emmy were ABC executives who were hoping that there'd be some residual hype for their new western.

"The Young Riders" also is interested in mining whatever mythology is left in the Old West. But if Larry McMurtry was writing a coda to those myths, "The Young Riders" is trying to find a continuation.

Mythology is all fine and dandy, but covering old ground usually results in cliche rather than art or even good entertainment. "The Young Riders" borrows the dusty filters of Robert Altman's "McCabe and Mrs. Miller," which gives the show a quasi-realistic feel, but that's undercut by the program's unbridled romanticism. Or maybe it's the realism that undercuts the unbridled romanticism.

In either case, we're deluged by bad guys in black hats (David Marshall -- not a bad villain), young heroes riding along on their pony-express missions in slow motion, a kid whose name is Kid who stands up to a professional boxer in order to win money to buy his palomino; and mounds of dialogue like "I'm here to learn ya about backflips" and "You're wearin' that Colt a bit low, aincha?"

The speaker in both cases is the entertaining character actor, Anthony ("Harry O") Zerbe, who, as the riders' tough-talking, tender-hearted mentor, first emerges bathing in a horse trough. He's the only member of the cast with any charisma, which is the most fatal of flaws for westerns. A time slot opposite "Cheers" isn't great grounds for success either. Here's betting that viewers will let sleeping myths lie.

* PEACEABLE KINGDOM 8 P.M. WEDNESDAYS, CHANNEL 7

Talk about sleeping dogs. And seals. And lions. And everything else that one is accustomed to finding in zoos, because that's where "Peaceable Kingdom" takes place.

If those were the only characters here, the show might have half a shot, considering that PBS nature specials sometimes out-rate any shows that CBS has placed at 8 o'clock.

Alas, there are humans in "The Peaceable Kingdom," namely Lindsay Wagner, Tom Wopat and the writers who give them dialogue unfit for man or beast. One of Wagner's obnoxious kids tells her, "We're caged in a zoo like the rest of your animals."

And that's the least of her troubles. "Peaceable Kingdom" adds woe upon woe, most of them financial, until we're ready to volunteer our services to help the poor animals. In return for Wagner and Wopat shutting up and CBS taking the show off the air.