The possibility that serotonin plays a role in prolactin secretion after electrical stimulation of the rhesus medial basal hypothalamus (MBH) was investigated. Prolactin responses to electrical stimulation and intravenous injection of 0.5 and 1.0 µg of thyrotropin-releasing hormone (TRH) were evaluated before and after administration of methysergide (MES), a serotonin receptor blocker (2 mg orally every 12 h for 48 h), and bromocriptine (CB-154), a dopamine agonist (2.5 mg orally every 12 h for 48 h). Both electrical stimulation and TRH caused prompt increases in serum prolactin. Prestimulation (basal) prolactin levels in both drug-treated groups were not significantly lower than basal levels in control groups. Pretreatment with MES significantly attenuated the electrically induced release of prolactin but had no effect on the TRH-induced release; CB-154 blocked prolactin release induced by both types of stimulation. The study reported here has provided evidence of a possible role for hypothalamic serotonin in releasing pituitary prolactin