Three American scientists won a Nobel prize yesterday for exploring the brain and showing how its powerful hormones govern the body.

The 1977 Nobel prize in physiology and medicine was awarded in Stockholm to Dr. Rosalyn Yalow of the Bronx Veterans Hospital in New York City, who will get half of the $145,000 prize, and Dr. Roger C. L. Guillemin of the Salk Institute in San Diego and Dr. Andrew Schally of the New Orleans Veteran Hospital, who will share the half.

All, said the Nobel committee, have opened "new vistas" that are leading to "formidable" developments in understanding disease.

Both Yalow and Schally are Vetarans Administration career research scientists, snd VA Administrator Max Cleland in Washington immediately congratulated them as "giants." Some parts of the VA medical system have been under recent attack, and Dr. John Chase, VA medical director, was equally quick to call the awards "the greatest recognition ever accorded the VA medical research program."

Yalow, 56, a physicist and na- tive of New York, is also the second woman to receive the medical prize and the to win a Nobel award this year. Gerty T. Corl of St. Louis shared the 1947 medicine prize with her husband, Carl. On Monday the Nobel committee awarded the 1975 peace prize retroactively to Northern Ireland peace-seekers Betty Williams and Mairead Corrigan.

Yalow's work involved much of the body, not just the brain, and she was recognized in large part for developing a method of analysis called radio-immunoassay [WORD ILLEGIBLE] that has revolutionized the chemical understanding of human physiology. Likewise, Guillemin's and Schally's work on the brain has affected much of the body.

But the brain has been a meeting ground for all three.

Guillemin and Schally showed that the pituitary gland at the brain's base - long called the body's "master gland" because its homones or chemical secretions regulate hormone production or function in many organs - is not really master but is controlled by brain hormones.

Specifically, these true master hormones are made in the [WORD ILLEGIBLE] mus, a kidney-bean-sized region of nerve cells also located at the base of the brain, though above the pea-sized pituitary.

Guillemin, 53, was born in Dijon, France. Schally, 50, is a native of Wilno Poland. Both are U.S. citizens, which helps maintain recent American domination of the science prizes. This is the third straight year the medicine prize has gone to Americans.

Both Guillemin and Schally, in interviews yesterday gave credit to Dr. Geoffrey Harris of Oxford University, who died in 1970. He first theorized that the hypothalamus made hormones.

Similarly, Yalow told how her assay method was developed in 20 years' work alongside Dr. Solomon Berson. He died 5 1/2 years ago.

Largely independently, the two learned the identity of brain peptides, or chains of amino acids, which control the pituitary gland that in turn controls one's reactions in stress, among other functions.

The pituitary also controls the thyroid gland, which governs growth and metabolism. In 1969 the Guillemin team synthesized the peptide TRH (thyrotropic releasing hormone), which controls thyroid secretion. In 1970 Schally isolated and synthesized the related LH-RH (or LRH), which affects female ovulation or egg production.

This new understanding, Guillemin said, could lead to ways to treat diabetes, infertility and mental disorders by adjusting body chemistry, as well as providing new chemical birth controls.

But in 1969 when he sent some of his findings to a leading American scientific journal, the report was rejected after a scientific referee implied that hypotalmic hormones were probably imaginary.

Yalow was honored for her discoveries about hormone production and how abnormalities cause RIA disease. As example, she helped doctors learn by using her RIA test that adult diabetics, unlike diabetic children do not have an insulin deficiency in their bodies but have a still unexplained inability to use insulin to control their blood sugar level.