Medical scientists are about to begin testing a possible new contraceptive -- part of a class of synthetic brain chemicals that could supplant today's birth control pills for women and provide the first such pill for men.

Thorough tests for human safety and effectiveness must still be done. At best, the scientists say, the new female pill might become available in the mid-1980s, and the male pill a few years later.

"We want to go very slowly," Dr. Samuel Yen of the University of California at San Diego said yesterday. He said the present pill for women was "applied very rapidly, and later we found a series of very serious complications."

Other scientists speaking to reporters at the National Institutes of Health were equally cautious. Yet all made clear that they are excited about the fertility-regulating potential of a brain chemical called "LRF" and some manmade near copies that have proved as much as 140 times more powerful in tests in animals and some men and women.

Women are not satisfied with present contrceptives, said Dr. Gabriel Bialy, contraceptive development chief at NIH's National Institute of Child Health and Human Development.

Versions of the same pills tried in men have proved unsatisfactory, working in some and not others, he added.

"What we have now," he reported, "is an entirely different chemical or spectrum of chemicals" that reproduce the brain's role in regulating both male and female sex organs.

The role has been clarified only recently. A part of the brain called thy hypothalamus produces a series of hormones, or peptides, that stimulate the pituitary gland at the base of the brain.

One is LRF (for "luteinizing releasing factor"). It stimulates the pituitary to make two more chemials -- LH (for luteinizing hormones) and FSH (for follicle-stimulatine hormone) -- which directly affect the male testes and female ovaries.

LH causes production of the key male sex hormone, testosterone. In women it produces the corpus luteum, a mass in the ovary that makes a female sex hormone during pregnancy.

LRF, then, is a master sex regulator. In the past five years, scientists at San Diego's Salk Institute and elsewhere have studied it and made the synthetic versions, with slight changes for special purposes.

In rats, monkeys and apes, the scientists said, LRF has proved its potency. And in tests on "hundreds" of men and women it has helped many who were infertile or sexually underdeveloped.

The new synthetic analogs have been tried in small numbers of men and women for up to four weeks. They have been safe so far and have given hints of th same powerful activity seen in animals. Together with Dr. Roger Guillemin, Salk Institute scientist who shared a 1977 Nobel Prize for his work on the hormone, "I was the first to try it," said Yen. "To see if it caused death. Or excitement."

The dose that he took caused neither. But "a huge amount of work" is needed, he said, to determine proper doses and ways to give the drug, ways that may have to thread a fineline between dangerous over-dosage and safe regulation of sexual chemistry.

NIH has spent $3.5 million so far, mainly on development of synthetic LRFs, Bialy said. Now it will spend anothers $1.75 million for more such work and two contracts to begin testing safety and physilogical activity in 124 men and women at the University of California in San Diego and Vanderbilt University, Nashville.

If all goes well, the first test of synthetic LRF as a human contraceptive could begin in women in three years, "but I won't say that someone won't start someplace else sooner," said Vanderbilt's Dr. David Rabin.