State Sen. Adelard L. (Abe) Brault, the senior Northern Virginia legislator, said today he will not seek reelection this fall, a decision that is likely to dilute the Washington suburbs' influence in the General Assembly.

"I don't mean to say it's the end of the world for Northern Virginia," said Brault, who will be 74 in April and has been in the legislature for 17 years. "But it will certainly have an effect . . . . That loss will be felt . . . and it will be some time before somebody in Northern Virginia has that kind of influence."

It was an assessment that one of Brault's colleagues shared. "I regard it as an enormous loss to Northern Virginia," said Sen. Joseph V. Gartlan (D-Fairfax), citing Brault's ability to persuade the legislature to appropriate money for Metro and Northern Virginia's colleges. "Without that influence we're not going to be as strong a delegation . . . . We'll have to look to Del. Dorothy McDiarmid and Sen. Clive DuVal," he said, referring to the Fairfax Democrats who are next most senior on the region's delegation.

"You can look at me and see why," said Brault of his decision. "I think the time has come to relinquish this responsibility to a younger person."

The short, feisty Fairfax Democrat, a member of the powerful Senate Finance Committee and chairman of the Education and Health Committee, plans to divide his time between his Florida condominium and his Northern Virginia law practice. He said he made the decision to retire several months ago at the urging of his wife.

He had planned to announce his decision at the end of the session, but "last month so many people contacted me and asked me to run again so I felt I had to be honest with people and tell them now."

Three years ago Brault was deposed as Senate majority leader, a post he had held for four years, by State Sen. Hunter B. Andrews (D-Hampton), who was supported by conservative legislators. "That was the greatest disappointment in my political life," he said today, "but it was not the end of the world."

Brault's announcement is expected to touch off considerable jockeying. His aide, Emilie Miller, former Fairfax Democratic Party chairman, plans to announce her candidacy soon and Brault said today he would support her. Others who have been mentioned as possible candidates are Democratic Supervisor James M. Scott and former Republican Dels. John H. Rust Jr. and Gwendowlyn Cody, both defeated last year.