Donald Kennedy announced yesterday his resignation as head of the Food and Drug Administration, a development that could impair quick passage of federal drug law revision legislation.

Kennedy, 47, will leave FDA, which began in April 1977, Kennedy established himself as an articulate and vigorous spokesman for his agency in dealings with the parent Department of Health , Education and Welfare and with the public.

The first impact of his departure may be felt on Capitol Hill, where public health subcommittees are expected to take up proposed major changes in drug regulation-changes supported by Kennedy.

"I'm sorry to see him leave," said Rep. Henry A. Waxman (D-Calif.), chairman of the House subcommittee."He gave leadership to the legislation to revise the drug laws and I don't know if his successor will want to embrace those same changes."

Waxman's Senate counterpart, Edward M. Kennedy (D-Mass.), was described by an aide as saddened that "a first-rate commissioner" was leaving FDA, but the aide said he did not foresee trouble for drug reform on that score.

"The capability of the agency is somewhat impaired now. It is a double blow, with Kennedy leaving and Richard Vodra, who drafted the administration bill, having gone already," the aide said.

The Senate subcommittee, however, expects to pass a drug bill by fall, at which point Waxman's panel will take it up. Waxman said yesterday he does not anticipate final action until next year.

Commissioner Kennedy said in a telephone interview that HEW Secretary Joseph A. Califano Jr. is "firmly committed" to the bill and that he is the "most powerful advocate" within HEW.

"He is very committed to that and he seized on the issue very early-charged us up - when we came up with the legilation," Kennedy said.

The commissioner was recognized as one of the stronger personalities within the HEW hierarchy, saying what he believed even if it rankled feelings in Califano's office.

One source indicated the differences that existed between the two men at one time had been resolved into "a good understanding" and that Kennedy was leaving solely because of the chance to return to Stanford.

In his letter of resignation to Califano, Kennedy said he had decided to leave after "long and difficult deliberation" and that the move was "determined by circumstances there and not here."

Kennedy added yesterday that he felt "the timing is really unfortunate. I had intended to stay through a presidential term and I don't like leaving early.But the position at Stanford will not wait."

He was a professor of biology there before his appointment to the FDA job.

In accepting his resignation, Califano said Kennedy had given "a remarkable demonstration of the effect that an individual with great talent and commitment can have on an organization."

That view was shared by consumer groups, which generally praised Kennedy during his time at the agency.