The ability of ovarian steroids to affect luteinizing hormone secretion is closely related to the influence of these steroids on the activities of several neurotransmitter systems within specific areas of the hypothalamus and associated brain areas. The purpose of this study was to characterize in vitro progestagenic effects on serotonin (5-hydroxytryptamine, 5-HT) and gonadotropin-releasing hormone (GnRH) release from hypothalamic slices from estrogen-primed, ovariectomized rats. Results of this study show that (1) progesterone can stimulate in vitro GnRH and 5-HT release from hypothalamic tissue slices of ovariectomized rats primed with estrogen and (2) the 5-HT receptor antagonist mianserin blocks the ability of progesterone to augment in vitro GnRH release from these tissue slices. This suggests that the influence of progesterone on the estrogen-induced LH surge is, at least in part, via progestagenic release of 5-HT and the subsequent effect of this neurotransmitter on the release of GnRH within the hypothalamus.