Harvard President Bok Resigns After 20 Years : Education: He broadened the university’s reputation and endowment during stormy years of protest.

Harvard University President Derek C. Bok, who during two tumultous decades broadened both the reputation and endowment at the nation’s oldest university, today announced his resignation.

“I haven’t yet had time to think about what to do next,” Bok said in a statement.

Bok, who became president in 1971, said that “it is time for me to step down and allow a new president to provide fresh energy and continuity of leadership through the next decade.”

Peter Costa, a spokesman for the university, said he could not elaborate on Bok’s reasons for the resignation, which will take effect at the end of the 1990-91 academic year.

Bok, a Stanford University graduate, earned a law degree from Harvard in 1954. He joined the Harvard Law School faculty in 1958 and became dean of the school in 1968.

Bok, 60, is a fellow of the American Academy of Arts and Sciences, and author of several books, including one on labor law that he wrote with Watergate prosecutor Archibald Cox.

During his tenure as Harvard’s president, Bok established both the John F. Kennedy School of Government and revived an undergraduate core curriculum based on the study of classics.

Bok often made headlines with his conservative views on education and business, including a controversial stand against a movement for total divestment of the university’s holdings in stocks of companies doing business in South Africa.

Among his challenges, he headed the school during sometimes violent anti-Vietnam War demonstrations, and negotiated over the years with black students and professors arguing for better minority representation.

But most of Bok’s efforts were low-key as he worked behind the scenes to maintain traditions at Harvard, which celebrated its 350th anniversary in 1986 and has one of the nation’s richest endowments.

There had been wide speculation in academic circles that Bok would step down following the celebration marking the Cambridge institution’s founding.

Bok is the son of a Pennsylvania Supreme Court judge, and his grandmother was the daughter of H. K. Curtis, founder of Curtis Publishing Co., which published the Saturday Evening Post.

He married Sissela Myrdal, the daughter of Swedish sociologist Gunnar Myrdal, in 1955. He and his wife, who has a doctorate in philosophy from Harvard and teaches at Brandeis University, have three children.