William B. Bryant steps down as chief judge of the U.S. District Court in Washington today, his 70th birthday. The occasion is a quiet turning point in his long and much heralded career during which he became the second black appointed to the federal bench here and the first to become its chief judge.

Bryant -- who is succeeded by Judge John Lewis Smith Jr. as chief judge -- is not looking back to see where he has left his mark. He doesn't think in those terms, he says.

"Sometimes I tell my children that I guess we are equipped to move forward and you don't have much time to be thinking about what happened last week," Bryant said in an interview in his chambers in the U.S. courthouse here.

The judge says he will leave it to history simply to record the things he has done in the long years since he was, by his own description, "the busiest poor lawyer in the city."

Bryant graduated at the top of his class in 1936 from Howard University Law School, but the only job he could get was as an elevator operator at $32 a month. He worked at other low level jobs, joined the Army and then came back to Washington in 1948 and began what would be a distinguished career as a criminal defense lawyer, often defending people who were too poor to pay him.

In 1957, Bryant successfully argued in the landmark Mallory case before the U.S. Supreme Court that the police may not use a confession extracted from a defendant during an unnecessarily long delay between his arrest and his first appearance before a judge.

Bryant was appointed to the federal court in 1965 and since then has left his imprint on an array of well-known court cases. He once ruled that the professional football draft was illegal, and he was the first judge to order President Nixon to turn over White House tapes in connection with a civil case.

Last year, Bryant presided at one of the controversial Abscam bribery trials involving former Florida congressman Richard Kelly, and during a bench conference told government prosecutors their case "has an odor to it that is absolutely repulsive."

Bryant also was the judge at the trial of two former high ranking FBI officials convicted of approving secret illegal searches of the homes of friends and relatives of members of the radical Weather Underground organization during the Vietnam war era. In an extraordinary bit of courtroom drama, Nixon appeared as a witness at that trial. The two former officials were pardoned by President Reagan this year.

Although he turns the chief judgeship over today to Judge Smith, Bryant says he is not ready to retire and plans to keep an active trial calendar.

Bryant first became chief judge in 1977, an automatic move based on his seniority on the bench.

Judge Smith takes on his second chief judgeship, the first having been in 1959, when President Eisenhower nominated him to head what was then was called the Municipal Court here and later the Court of General Sessions. In substantially reorganized form, it now is called D.C. Superior Court.

Smith was appointed to the federal bench in 1966. He ruled that Nixon and two administration officials violated the Constitution when they wiretapped a former National Security Council aide's home telephone.