In five normal women, the expected chlorpromazine-induced rise in serum prolactin was inhibited by pretreatment with a serotonin antagonist, cyproheptadine. This indicated that the serotoninergic system has an excitatory effect on the release of prolactin. When similar studies were done in nine women with hyperprolactinemia and galactorrhea, the administration of chlorpromazine did not consistently produce a rise in serum prolactin, and pretreatment with cyproheptadine did not have a predictable effect. In most cases, this chlorpromazine-induced reduction in the prolactin inhibitory factor could not further increase the secretion of prolactin, so that there was no rise to inhibit.