City Council member Marion Barry started it by using an exclusive interview to announce his plans to announce later that he would be running for mayor.

Council Chairman Sterling Tucker followed suit, calling a press conference last month to announce that he, too, planned to announce his mayoral candidacy at some uncertain future date.

Yesterday, Ronald H. Brown, chairman of the board of the University of the District of Comumbia, introduced a slightly different approach into the sometimes confusing affairs of preelection politicking surrounding the Sept. 12 primary election.

Brown hastily summoned reporters to his office to announce that he would not announce his candidacy for mayor, even though he had never announced that he intended to announce it in the first place.

"As many of you know, rumors and speculation have circulated about as to my political plans," said Brown, who had for months fueled those rumors by volunteering his own name as that of a possible "new face" candidate and sounding out possible political and financial support.

"I have carefully weighed the pros and cons of seeking the office of mayor," he said, "and have decided that I will not now seek that or any other elective office."

Brown's self-elimination from the field of candidates left without a candidate for the moment supporters of Clifford L. Alexander, the present Secretary of the Army who had run unsuccessfully against Mayor Walter E. Washington in 1974. Many of Alexander's former political associates had been among those urging Brown to run.

Brown said yesterday that among the factors he had considered were his role as board chairman of the University, which is currently merging three formerly independent schools into a single university, and his full-time job as deputy executive director of the National Urban League.

He said he believed he could be of most service to the city by remaining as UDC board chairman and continuing his work at the Urban League - two posts that he said he would have quit if he decided to run for mayor.

A knowledgeable source close to Brown said that the decision not to run was in large part prompted by the realization that Mayor Washington, who has still to announce his political intentions, would probably seek re-election.

Brown and his supporters believed that the 34-year-old Washington native could have done well against Tucker and Barry. With Washington in the race, however, many of those likely to support Brown would instead turn to Washington, the source said.

Brown's decision narrows the field of announced, informally announced or likely candidates for the Democratic mayoral nomination to five. In addition to Tucker, Washington and Barry, they include lawyer John L. Ray and teacher Charles Suel (Trummie) Cain.