Popkin said he plans to rest for several days, but that he should return to teaching his courses next Tuesday.

'As Prison Go'

He said that he was in "a good prison as prisons go, so I've learned what's wrong with prisons as a whole." He added half-jokingly that he would not comment extensively on conversations he had had in prison until he had checked for the existence of a grand jury investigating prisons in Norfolk County. He said that he considered his imprisonment an experience among friends and "not a scholarly event."

Susan Shirk, Popkin's wife, said yesterday that she was "surprised at his release," and that she fully expected him to be imprisoned until January 12, the grand jury's full expiration date. "It certainly will be a nicer Christmas," she added.

Stemer yesterday praised the government for what he called "wise restraint" in its decision. "The decision was the government's and not ours," he said, "but we are pleased they reached that decision."

President Bok, in a short statement, said that Popkin's jailing had been a matter of continuing and extreme concern to the Harvard Administration."

"I am sure the Harvard uncertainly joins in my happiness over his release from jail," Bok said.

Popkin was jailed for refusing to answer three of an original 14 questions asked by the Boston grand jury in its March 27 session. Two of those questions sought to determine whom he had interviewed to gain his knowledge of the participants in the Pentagon Papers study. A third asked if he had discussed the content or existence of the papers between January 1 and June 13, 1971