French-born American physiologist whose researches into the hormones produced by the hypothalamus gland resulted in his being awarded a share (along with Andrew Schally (Schally, Andrew V.) and Rosalyn Yalow (Yalow, Rosalyn S.)) of the Nobel Prize for Physiology or Medicine in 1977.

Guillemin was educated at the universities of Dijon, Lyon, and Montreal. He taught at the Baylor College of Medicine in Houston, Texas, from 1953 to 1970, except for the years 1960–63, when he was on the faculty of the Collège de France in Paris. From 1970 he was a resident fellow and research professor at the Salk Institute for Biological Studies in La Jolla, Calif. Guillemin became a U.S. citizen in 1963.

Guillemin proved the hypothesis that the hypothalamus releases hormones that regulate the pituitary gland. Among the hypothalamic hormones that he and his colleagues discovered, isolated, analyzed, or synthesized were TRH (thyrotropin-releasing hormone), which regulates thyroid activity; GHRH (growth hormone-releasing hormone), which causes the pituitary to release gonadotropin; and somatostatin, which regulates the activities of the pituitary gland and the pancreas. Guillemin also discovered an important class of proteins called endorphins that are involved in the perception of pain.