No one ever said dieting was good for the disposition. But when Dr. Robert Atkins and Nathan Pritikin, two acknowledged heavyweights in the field of telling other people what not to eat, met Wednesday morning on the "Tomorrow" show, they went after each other with all the delicacy of three-alarm chili.

Later in the day, Atkins announced that he "planned to file" a $5 million law suit against Pritikin, charging him with libel and slander and leaving the uniniatiated aghast at the intensity with which hardball is played in the diet-book world.

Atkins, he of the high-protein, low-carbohydrate school of staying thin, said at a press conference he called yesterday that he gave Pritikin, author of "The Pritikin Program for Diet and Exercise," copies of court papers after the two had appeared on the "Tomorrow" show. "I was a gentleman, I waited till the show was over," he said. He also speculated that his colleague might feel "that to succeed with his rather ambitious projects he must destroy the credibility of other dietary projects that are in competition with him."

The Atkins-Pritikin feud has been raging for more than a year, but for those who came in late, a capsule summary was available at their meeting on the "Tomorrow" show.Pritikin charged, as he has in the past, that the Atkins diet can lead to cancer, heart disease and constipation, not to mention bad breath.

Atkins, meanwhile, countered on the show that Pritikin's low-fat, heavily vegetarian diet was so boring that it was hard for people to stay on it. ". . . If people want to go on Mr. Pritikin's diet and eat beans and peas to lose weight," Atkins said, "that's fine, you see, if that's the way they want to do it."

Not so fast, said Pritikin. "Our diet includes salmon, souffles, stir-fried steak, beans, peppers . . . and if you want a pineapple, lemon cheese cake or a chocolate-flavored mousse that you'll throw out your chocolate cake for, this is the diet to go on. You've never seen our 500 recipes. . ."

"We don't need recipes," countered Atkins. "We can go to a restaurant and order the best thing. . . We can go into an Indian restaurant, an Italian restaurant. We don't have to worry. We can go to fish, brisket, lobster in melted butter. We don't have to worry about recipes."

"It was hysterical," said a "Tomorrow" show executive who witnessed the confrontation between the dueling diet experts. "Basically, they both made each other look like asses. "We kept them in separate dressing rooms so that the sparks would really be flying by the time they were on camera. We were hoping for controversy."

And controversy they got. "From the beginning," said the executive, "their lips got very tight and there was a lot of tension. It was just a fun show," right up there, she said, "with the one we did with Jerry Falwell and Bob Guccione."

Round one began when Pritikin said, "First of all, I'd like to congratulate Dr. Atkins on recommending my diet as the ideal diet. I just quote out of his book here, and here's what he says on page 35."

Atkins, of course, denied saying anywhere that he recommends the Pritikin diet, and leapt swiftly to the offense, claiming that the Pritikin proposition was "marginal in protein." In the end, Tom Snyder, the show's host, gave each diet guru a chance to sum up his position. Atkins went first, but, it appeared to the executive, so incensed was he by what Pritikin had to say that only Snyder's hand over his mouth prevented him from leaping back into the fray with a rebuttal.

"Well, if you get really serious about this, it can put you into a house with six figures," said Richard Simmons, whose book "Never Say Diet" has been perched for months on the best-seller lists. "But I still live in my little apartment and I'm still into reality." Besides, Simmons said airily in a telephone interview, "I don't think the only thing that helps people is just a dumb diet. You need exercise as well." oSimmons' exercise show is syndicated in 91 television markets.

Simmons says he has tried both the Atkins and Pritikin approaches to weight loss and, mirabile dictu, doesn't seem to think much of either of them."It's the 'no' diet," he says of the Pritikin plan. "You have to say no to everything. I don't even think he looks that healthy. I certainly wouldn't invite him to a dinner party." As for Atkins, he said, "I don't know what he thinks. I think he just wants another book on the best-seller list."

Pausing cheerfully to knock another competitor ("If you follow her diet you'll end up looking like a very loose guava that no one will buy"), Simmons concluded by extolling the features of his next book. "These guys love to be at each other's throats," he said.

Judy Mazel, whose book, "The Beverly Hills Diet," is resting comfortably among the best sellers, sighed when she heard about all the commotion. "I find all this competition among diet people, like, so foolish," she said. "I mean, what's the point? There's enough fat to go around. Over 90 percent of the people in this country need to lose weight."

After all, said Mazel, "we're trying to help humanity. That's what it's all about. I know. I came from a very fat place. But I discovered the key to fat. The secret is slim. And what I'm trying to get out of all of this is to help people out of their pain."

Mazel's method, she said, involves "utilizing the enzymatic potential of fruit, so that you can have your cake and eat it too, even if it's cheesecake, so that you can get away with murder and not serve a life sentence, so that you can have humburgers and hipbones, cheesecake and cheekbones, so that diet doesn't mean deprivation.

Pritikin was not available for comment.