Neutralized acidic extracts of rat hypothalamic tissue stimulated release of follicle-stimulating hormone (FSH) from adult male rat pituitary tissue during a 6-hr. incubation period. A graded response was observed in release of FSH by pituitary tissue in response to doses of extract ranging from 1/32 to 1/4 hypothalamic equivalents per incubated pituitary. Evidence was obtained that castration increases and testosterone propionate injections reduce the FSH-releasing activity of hypothalamic extracts.