The number of black students entering the nation's law and medical schools has begun to decline slightly after almost a decade of rapid growth.

Officials of the law and medical associations that reported the enrollment declines attributed them partly to greater opportunities for blacks in other fields, such as business and engineering.

They said there also appears to be less willingness by admissions officers to accept students with poor academic records than there had been in the late 1960s and early 1970s, when affirmative action programs began.

Educators who have pushed strongly for expanding minority enrollment said the downturn may have occurred because black students are discouraged by the legal controversies surrounding preferential admissions policies.

"There's no doubt that the Bakke case has had a chilling impact on applications by minority group students," said Wade j. Henderson, executive director of the Council of Legal Education Opportunity, a federally funded program to boost minority enrollment in law schools.

"There's a decided pessimism that they are no longer viewed (by admissions officers) with the eagerness they were even a year ago."

According to the Association of American Medical Colleges, blacks accounted for 6.7 percent of first-year medical students last fall, compared with peak of 7.5 percent in 1974. The actual number enrolled-1,085-was 21 fewer than it had been in 1974.

In law schools, blacks made up 4.9 percent of last fall's first-year class, compared with a peak of 5.3 percent in 1976.

The American Bar Association, which reported the figures, said the number of blacks enrolled last fall was 1,945.5, down from 2,128 a year earlier.

The ABA tabulation indicated that the reason for the fraction in the reported number of blacks was that one student identified himself as half-black and half-Asian.

"I think it's a silly way of reporting things," said Millard H. Ruud, executive director of the Associaton of American Law Schools. "But it's based on self-definition.That's very important to applicants. Sometimes it's either you're in or you're not in because of it."

According to a study last year by Educational Testing Service, only about one out of four blacks admitted to law schools would have been accepted, based on academic records and test scores, if their race hadn't given them a boost.

The study was cited bu the law association in its brief to the U. S. Supreme Court defending affirmative-action programs.

Last June the high court ruled that such programs were permissible as long as they do not establish strict quotas. However the court ordered that Allan Bakke, a white man who charged that he had been discriminated against be admitted to the University of California medical school at Davis because admissions there used a quota system.

In 1968, just before most affirmative action programs started, blacks accouted for 2.7 percent of the first-year students in medical schools and 3.1 percent in law schools.

Ruud said some of the black students admitted during the early years of the program had "such long odds against them" because of their poor academic records that the law schools "may have been satisfying our own consciences but not doing anything for the young people involved except adding another failure."

He said black students admitted more recently have been better prepared.