Mediobasal hypothalamic (MBH) slices of adult ovariectomized (OVX) rats with or without 17β-estradiol (E2) pretreatment, were superfused in buffered (pH 7.2) oxygenated Locke medium containing bacitracin. Pulsatile or continuous administration of progesterone (10–7 or 10–8M) produced a marked increase in luteinizing hormone-releasing hormone (LHRH) release provided the animals had received E2 prior to sacrifice. Omission of Ca++ in the medium, or addition of a Ca++ channel blocker (D-600, 10–4M), of a calmodulin inhibitor (trifluoperazine, 30 µM) or of a calmodulin-dependent tubulin kinase inhibitor (phenytoin, 50 µM), antagonized the stimulatory effect of progesterone. When sodium channels were blocked by tetrodotoxin (5 × 10–7M), the stimulatory effect of the steroid was completely abolished. The amplitude of the K+-induced LHRH release was slightly increased in the presence of progesterone (10–7M) but only from MBH slices of OVX-E2-treated rats. These results indicate that the secretory response of LHRH to progesterone requires priming with estradiol, is Ca++-dependent and involves mediation of calmodulin and a calmodulin-dependent