Intracarotid injections of PGE1 into castrated-estrogen-pretreated female rats are followed by a rapid and prolonged release of LH, but have no important effects on FSH secretion. Intracarotid injections of PGF2α into the same type of animals induce a biphasic effect on LH release, and a marked and sustained hypersecretion of FSH. PGE1 given intracarotidly in normal male rats results in an elevation of plasma LH characterized by the occurrence of two peaks, but does not alter plasma FSH. Intracarotid injections of PGF2α in male rats have no effect on plasma LH levels, and cause a delayed elevation in plasma FSH. These results provide the first direct demonstration that PGs may induce the release of pituitary gonadotropins.