Columnist William F. Buckley Jr. leaned back in his chair, just as he does on his television show, "Firing Line." But in U.S. District Court here yesterday, his antagonist, attorney Mark Lane, had considerable control of the format.

Buckley and his conservative magazine, The National Review, are pressing a $16 million libel suit against the right-wing Liberty Lobby and its founder, Willis A. Carto. Lane, whose career started on New York City's left, is defending them with accusations that National Review itself is "profascist."

After Lane broke into Buckley's testimony with repeated objections, Buckley remarked, "I'm terribly constipated by these interruptions."

"There are certain rules and traditions that we follow in the courtroom, Mr. Buckley," Judge Joyce Hens Green declared. "Please follow [them]."

Later, answering questions from Lane, Buckley vehemently denied that he had ever "slurred Negroes."

"If I believed in slurring Negroes, I wouldn't be suing Liberty Lobby," Buckley continued. "I would be subscribing [to the group's weekly newspaper, Spotlight]." TWhen Lane objected to the remark, Green warned Buckley "to refrain from that kind of editorial comment," but she said Lane should "just ask the questions with no frills added."

As the trial completed its second day, Buckley denied the four statements in Spotlight that are the subject of the libel suit: that National Review favored giving "militant sex deviates . . . the right to molest" children; that the magazine was a "mouthpiece" of the Anti-Defamation League of B'nai B'rith; that Buckley had a "close working relationship" with American Nazi Party founder George Lincoln Rockwell, and that National Review had printed a "muddled smear" of Liberty Lobby in collaboration with a John Birch Society member and Rabbi Meir Kahane of the Jewish Defense League.

In 1983, Green granted summary judgment for National Review on the first two counts. She also dismissed Liberty Lobby's libel suit against National Review that started the drawn-out case. Liberty Lobby's claim had stemmed from a 1979 article stating that Carto had funneled money to Lyndon H. LaRouche Jr., head of the National Caucus of Labor Committees, which has turned from left-wing to right-wing during the last decade.

The six-member jury has been asked to decide the two remaining libel counts against Liberty Lobby and to set damages on all of the libel charges.

Yesterday Buckley acknowledged, as he has previously, that he once hired Rockwell to solicit subscriptions for National Review about two years before the American Nazi Party was founded. But Buckley also read a letter he wrote in 1958, telling Rockwell that he was "physically appalled" by Rockwell's praise for the "despicable Hitler."

Buckley said he had commissioned a 1971 National Review article criticizing Carto and Liberty Lobby as anti-Jewish and anti-black because he felt they were "irresponsible and were damaging the conservative cause." He added: "It is and was and always will be one of the missions of National Review to ward off the fever swamps of the crazed right."

Responding to Lane's accusation that National Review was "racist," Buckley acknowledged that he had written a 1957 editorial calling whites in the South an "advanced race." But Buckley explained that he doesn't believe in the universal franchise and said that "no qualifying standards [for voting] should be applied to blacks that are not applied equally to whites."