When Kenan Thompson and Kel Mitchell get together, atoms bond and sparks fly.

They have chemistry - an electric connection that Nickelodeon is using not only to supercharge its Saturday night lineup but also its new movie division.

Like two giddy kids in a science lab, Thompson and Mitchell make comedy that is greater than the sum of their collective one-liners. Theirs is a partnership that summons memories of the great comic teams of the past.

Nick fans first met Thompson and Mitchell early last year on the network's comedy sketch show All That.

Now in the sitcom Kenan and Kel, which will have a preview tonight, Nickelodeon is giving the comedians a half-hour vehicle to strut their stuff.

Early next year the network expects to begin production of a feature film in which Thompson and Mitchell will play two characters created for All That.

Landing at Nickelodeon at the right time and place - just when both television and movie production were being ramped up - Thompson and Mitchell are poised to be the network's biggest stars should their humor hit the right chord with Nick's young viewers.

Nickelodeon officials have confidence. Normally, the network creates a show concept and then goes out to find the actors. Kenan and Kel is the first Nickelodeon show launched to showcase particular talents.

"One of the breakthrough elements is the uniqueness of making a show that is based on the talent rather than the other way around," said Cyma Zargahami, general manager/senior vice president of programming at Nickelodeon. "This idea came out of the fact that these two had such great chemistry together."

Although Thompson and Mitchell were chosen without regard for their heritage, as the first black stars of a Nickelodeon prime-time sitcom they could help broaden the network's demographic appeal.

Kenan and Kel will be a change of pace for Nick's target audience - children ages 6 to 11. But parents who watch the show will find it familiar. For Thompson, in many ways, is playing a 15-year-old Ralph Kramden, and the rubber-faced Mitchell is a latter-day Ed Norton.

Nickelodeon indeed is promoting the buddy comedy as a modern-day Honeymooners for kids. Harking back to the '50 series, each episode of Kenan and Kel will begin with dialogue in front of a stage curtain.

The actors play best friends in Chicago who, to hilariously disastrous results, do more than daydream about grownup pursuits.

In tonight's preview at 8, Kenan and Kel try to buy a car. In another episode they sue the world's biggest tuna company, and in another Kenan sets out to market his cure for the common cold.

Thompson is straight man and Mitchell is physical. Slapstick? You bet, and it's an art form that both the comedic actors have studied. Thompson, an Atlanta 18-year-old whose movie credits include D-2 Mighty Ducks, is Bud Abbott to Mitchell's Lou Costello; Mitchell, an almost-18 Chicago native whose big break was All That, is Jerry Lewis to Thompson's Dean Martin.

Nickelodeon will devote its prime-time hours tonight to the network's prized new talents. The preview of Kenan and Kel will be followed by three episodes of All That, all handpicked to feature Thompson and Mitchell's best sketches. After the first All That episode, the network will air a music video of rapper Coolio's Kenan and Kel theme song, "Aw, Here it Goes" and a short feature on the making of the video.

The series will begin airing regularly with an Oct. 11 premiere.

Thirteen episodes of Kenan and Kel were produced before a studio audience at Universal Studios Florida between April and Aug. 9. The show will return to production there next spring. All That, produced for two seasons in Orlando, will begin production for the first time this fall in Los Angeles, in part to exploit opportunities to bring on more celebrity guests.

Thompson and Mitchell, who will continue as All That cast members, are animated and energetic on stage but more serious and quiet when off. Both say they feel fortunate for their new opportunities and to have found each other.

"When we get together, it's chaos, it's hilarious," said Mitchell.

Kenan and Kel is in many ways a tribute to the success of All That, a critically praised Saturday Night Live for kids created in part to help find talent for Nickelodeon.

All That producers were referred to Thompson - whose mother put him in acting classes at about age 10 on the recommendation of a friend - by the director of D-2 Mighty Ducks. Thompson also will have a role in the Mighty Ducks sequel to be released in October. His other credits include the movie Heavy Weights, CNN's Real News for Kids, and the ABC kid specials Night Crawlers and T-Rex.

Unlike Thompson, Mitchell made his TV debut with All That. The show's producers discovered him in their Chicago auditions. Mitchell's mother had enrolled her son - always the class clown - into a community theater program at the age of 11, in part to keep him off the streets and out of the way of gangs.

Thompson and Mitchell thus came to All That with years of

theatrical training and, as fate would have it, both had old-men characters in their repertoire. One of their first and most popular pairings was as the creaky codgers, "Mavis and Clavis." Thompson and Mitchell just got better and better together, said Brian Robbins, co-executive producer of both All That and Kenan and Kel.

A former cast member of the school-based series Head of the Class, Robbins said he was eager to do something more original with his two talented comedians in their teen sitcom. Life in Kenan and Kel is a perpetual weekend. Kenan has a part-time job in a grocery store, and Kel is the constant companion.

Kenan dreams of making an early fortune, and he drags the reluctant Kel into his harebrained schemes.

The teenagers' hip-hop fashions and comic adventures are sure to make Kenan and Kel role models for a generation of children now growing up on the kids cable network. But Thompson doubts that their activities will be mimicked any more than another generation wanted to be in the chocolate factory with Lucy Ricardo.

"The stuff we do is so outlandish that no regular 15-year-old would do (it) because they would die," Thompson deadpanned.

Kenan and Kel fall deeper into trouble with each minute of the show.

"The show is like a time bomb," Mitchell said. "Tick, tick, tick, tick, tick, tick. And then it explodes, and it's like the greatest show you've ever seen."

Should young audiences agree, the result for Thompson and Mitchell will be a molecular chain reaction toward national celebrity.