FAMILY : On This Tour, the Medium Is the Mess : Smells like preteen spirit? Nah, parents get in on the action too, in Nickelodeon’s zany ‘Mega Mess-a-Mania,’ coming to L.A. this week.

For the uninitiated, “Mega Mess-a-Mania” is the ultimate in audience participation. Think “Let’s Make a Deal” meets “The Slime People.” First, thousands of wildly costumed, placard-waving hopefuls scream to be chosen to go onstage. After that, it’s not whether you win or lose, it is whether you get slimed.

An estimated 10,000 gallons of the green stuff will be on hand (and legs and arms . . . ) as Nickelodeon brings its “Mega Mess-a-Mania Live” Tour to Southern California this week.

“Typical Nickelodeon,” says one of the show’s hosts, Donnie Jeffcoat. That means crazy games, a lot of whipped cream and going “oogabooga nuts.”

The “Ghostbusters” movies created an image of slime as something to be feared. Nothing could be further from the truth, says Nickelodeon’s resident slimeologist and the show’s “slimeological consultant,” known only as the Professor. “Slime just wants to be your friend,” he says.

Slime is described as an organic matter from Earth, deposits of which just happen to be found at Nickelodeon’s studios in Florida.

“Slime cannot hurt you. It is just messy. It is quite an honor to be slimed,” the Professor says. “However, don’t come to the show in your Sunday best.”

“That’s the key,” echoes Jeffcoat, who hosts Nickelodeon’s “Wild and Crazy Kids.” [is this the TV show on which the mega mess-a-mania tour is part of/should we have a tag at the bottom on when the show airs on Nick?] “Slime washes off, but don’t come in clothes you really care about. Go buy a $2 orange T-shirt and wear an old pair of jeans. Because if we pick you, there’s a very good chance when you get up on that stage, you’re going to walk off with some sort of slime.”

Judging from a recent “Mess-a-Mania” performance here, the audience wouldn’t have it any other way. Several youngsters had the words “Slime me” painted on their faces. Others thrust homemade signs that read, “Pick me!” and “I live for slime.”

Some will go to any lengths to be noticed by Jeffcoat and co-host Bryan Stinson of Nickelodeon Studio’s Game Lab, who spring through the audience picking contestants. One, Jeffcoat recalls, painted his entire head in Nickelodeon colors, orange and green, and topped himself with sparkling glitter.

And that was an adult.

This, its creators say, is exactly the point of “Mess-a-Mania,” not just to encourage kids to be kids but adults to do so as well.

“We like to get people, young and old, down to their kid essence,” says Don Brandenburg, one of the show’s creators. “While the show fulfills a kid’s fantasies by allowing them to do something they wouldn’t normally be allowed to do, it was also designed to get kids and parents to play together. The great thing for me is watching families come onstage and, after their game, leaving arm in arm, almost like they have come through a war together.”

In the world of Nickelodeon, there is no such thing as an inner child. Young and old are encouraged to outwardly revel in the glory that is Gak, a Nick term for getting down and dirty. “Mess-a-Mania” even has its own Gakmeister, Darla Boogaire, who serves as the show’s mistress of ceremonies. It’s a messy job, but somebody’s got to do it.

Decked out in a beehive wig, outsize rhinestone glasses and bulbous nose, Boogaire is “a combination of Jane Hathaway from ‘The Beverly Hillbillies,’ Julia Child and Mrs. Doubtfire,” says Brandenburg, her alter ego. “I see her as everyone’s favorite aunt who has kept her sense of being a kid.”

As the six games that make up the show wildly illustrate, happy is the child who gets to plant a heaping, dripping whipped-cream pie into a parent’s face. And happy is the parent who is called upon to literally make a “horse’s butt” out of his or herself (part of “Mess-a-Mania’s” version of “Post Office”).

The games, Jeffcoat says, were designed “to allow kids to release their natural kid energy. But adults are also such a big part of the show. A lot of times they get even more excited than the kids, and their enthusiasm just makes their kids’ energy even higher.”

During “Clothes Out,” for example, kids race to deliver heaps of outrageously mismatched clothing to their parents, who must then put on as many of the items as they can while under a giant tarp.

Before a game makes it to the “Mess-a-Mania” stage, it is kid-tested and approved, Brandenburg notes.

“The ‘Pieatholon’ went through three different versions,” he says. “We just kept trying things until it worked. At one point in the evolution of ‘Clothes Out,’ one of the contestants pointed out that one team had an unfair advantage and we fixed that.”

Who is most likely to be chosen?

“Those who are energetic and have made an effort,” Jeffcoat says. But those who are just the opposite are just as likely to be picked. “If we see you’re just sitting in your chair,” he says with a laugh, “then you’re a good target. We had a guy in Portland, Me., a few weeks ago who was talking on his cellular phone. We gave him a little problem.”

But, Jeffcoat emphasizes, those who just want to watch the show can relax: “If we come up to you and you say, ‘No, thank you,’ we’ll immediately move on because we’re in a hurry. We would never press anybody, unless we can see it in your eyes that you’re just embarrassed. But if you genuinely have stage fright or don’t want to get messy, we will go on to someone else. We don’t have time to argue.”

One of the challenges of staging “Mess-a-Mania” is to make the show entertaining and cut down on the disappointment factor for those who are not chosen as participants. Boogaire handles much of the audience interaction, from absconding with parents’ shoes to posing for pictures. The show is further enlivened with appearances from such popular Nickelodeon characters as Grandpa from “The Rugrats” and Stimpy, as well as a mammoth game of volleyball played out in the audience with several giant beach balls.*

* “Mega Mess-a-Mania” plays Saturday at 1 and 4:40 p.m. at the Pond, 2695 E. Katella Ave., Anaheim. It also will be performed next Sunday at 1 and 4:30 p.m. at the Forum, 3900 W. Manchester Blvd., Inglewood. $11.50-$20. (310) 419-3257.