Mort Zuckerman surprised the magazine world yesterday by disclosing that he has sold the Atlantic Monthly, one of his prized media possessions, to Washington businessman David Bradley, who owns National Journal.

Zuckerman last night called the decision "a gut-wrenching experience," saying: "I have a family and I have an awful lot on my plate, eight different businesses. And I just can't keep up with everything and do what I want to do with my life." The real estate developer said he has no plans to sell U.S. News & World Report or the New York Daily News.

Bradley plans to replace William Whitworth, who has edited the Boston-based Atlantic for nearly two decades, with Michael Kelly, now the editor of National Journal, knowledgeable sources say. Kelly, a former editor of the New Republic, will be promoted at National Journal and continue to oversee the public-policy weekly as well, the sources said.

Bradley, 42, who bought National Journal from Times Mirror in 1997, is a little-known executive who owns the Advisory Board Co., a blue-chip Washington consulting firm that he founded 20 years ago. Bradley netted $142 million in February when another of his firms, Corporate Executive Board Co., went public with an initial stock offering. He launched that business from his mother's apartment at the Watergate.

Zuckerman, 62, said Bradley approached him through an intermediary and that he agreed to the sale--the price was not disclosed--because "I've been reading National Journal and frankly I think it is a very, very good magazine. They've done a hell of a job with it."

Bradley, who was traveling yesterday, said in a statement: "I proceed here with two parts modesty for every one part confidence. The honest question is how to be worthy of the Atlantic's recent history and long tradition." Whitworth declined to be interviewed.

John Fox Sullivan, president of National Journal, said Bradley had boosted the Washington magazine's budget by 50 percent. "His attitude is you make it as good as you can, spend money and the market will respond," said Sullivan, whose magazine has a circulation of 15,000. "He's not an egotistical maniac, like some other people. He's very smart."

Bradley is a native Washingtonian who attended Sidwell Friends School, earned an MBA at Harvard and attended Georgetown Law School. After Bradley bought National Journal Group, which includes the Hotline political digest, he hired such well-known journalists as Kelly, Stuart Taylor and Charlie Cook for National Journal.

Taking the helm of the 140-year-old Atlantic will cap a remarkable career rise for Kelly, a former reporter for the Baltimore Sun, New York Times and the New Yorker. Kelly, whose syndicated column is carried by The Washington Post, had never been an editor until he took over the New Republic, only to be fired nine months later after clashing with owner Martin Peretz. Kelly became editor of National Journal last year.

The Atlantic, whose circulation is 457,000, was the most stable journal in Zuckerman's empire, shielded from the constant reshuffling of editors at his other publications. Zuckerman hired Whitworth after buying the magazine in 1980. Its success "is all due to Bill Whitworth," Zuckerman said. "He is as close to a genius as anybody I've ever known in the field."

Zuckerman, who has a 2-year-old daughter with Marla Prather, the art curator he married three years ago, said his priorities have changed and he plans to cut back his workload. "I'm much more active in U.S. News and the Daily News. So if I have to choose, I would like to choose those where I'm much more active. I don't like to be a spectator. I like to be a participant."

CAPTION: David Bradley, owner of National Journal, bought Atlantic Monthly from Mort Zuckerman.