The intravenous administration of prostaglandin E2 (PGE2; 100 µg/rat) to immature male and female rats increased the serum level of luteinizing hormone (LH). This effect was prevented by the prior administration of an antiserum to the hypothalamic gonadotropin-releasing hormone. LH release from rat pituitaries in vitro was not stimulated by PGE2 (10 µg/ml). It is inferred that the stimulatory effect of PGE2, on LH release is not a direct one on the pituitary gland, but is exerted at the level of the hypothalamus.