The effects of reproductive hormones on pineal serotonin (5-HT) metabolism were examined in castrated rats. Castration of males but not of females increased 5-HT content, whereas follicle-stimulating hormone (FSH) and luteinizing hormone (LH) augmented, and testosterone (T) decreased, pineal 5-HT concentration of orchidectomized rats. In males castration slowed the rate of decline of pineal amine concentration following the injection of p-chlorophenylalanine (PCPA). The administration of T, FSH or LH brought about significant acceleration of pineal 5-HT turnover rate. In spayed rats estradiol treatment decreased 5-HT decline following PCPA. Pineal monoamine oxidase (MAO) activity type B (assaying by using β-phenylethylamine as substrate) was increased by T injection whereas estradiol diminished MAO type A activity (assaying by using 5-HT as substrate). The administration of prolactin to castrated male rats resulted in depression of both types of MAO activity while LH and FSH reduced the activity of MAO type B. The injection of T to rats subjected to superior cervical ganglionectomy counteracted partially the depressed pineal 5-HT levels caused by surgical manipulation. LH or FSH also augmented pineal 5-HT concentration in ganglion-ectomized rats. Bilateral decentralization of ganglia abolished hormone effects on amine levels. These results offer additional support to previous observations indicating that the pineal gland and its innervating sympathetic neurons are effector tissues for reproductive hormones.