A possible direct effect of prostaglandins E1 and E2 (PGE1 and PGE2) on luteinizing hormone (LH) release at the pituitary level was studied in vitro using anterior pituitary cells in primary culture, a system approximately 10-fold more sensitive to stimulation of LH release than previously used hemipituitaries. No effect of PGE1 or PGE2 could be detected on the time course of basal or LH-RH-stimulated LH release or on the LH responsiveness to LH-RH. This absence of a direct effect of PGEs at the pituitary level on LH release was confirmed by in vivo experiments using female rats under Surital anesthesia in the afternoon of proestrus. After intravenous injection, under these conditions, 15(S)-15-methyl PGE2 was 3-5 times more potent than PGE2 to increase plasma LH levels while PGE1 had about 50% the potency of PGE2. Injection of sheep anti-LH-RH serum one hour before PGE1 or PGE2 injection not only lowered basal plasma LH levels but prevented the rise induced by PGEs. These data indicate clearly that the increased plasma LH levels observed after in vivo PGE injection are secondary to a stimulation of LH-RH release while PGEs do not appear to have a significant effect on LH release at the pituitary level.