Effects of serotonin (5-HT) on serum gonadotropin (GtH) levels were studied in female and male goldfish, Carassius auratus L. Intraperintoneal (ip) administration of 5-HT (10 micrograms/g) produced a significant increase in serum levels of GtH at 0.5, 1, and 2 hr but not at 3-4 hours, after the drug was injected. Administration of different concentrations of 5-HT into the brain third ventricle (iv) did not affect serum GtH levels. In both sexes, the intraperitoneal administration of different doses of 5-HT (2.5-20 micrograms/g) caused a dose-dependent increase in serum GtH levels in fish at different gonadal stages; the greatest responses occurred in mature (= prespawning) fish. Fluoxetine (10 micrograms/g; ip), a 5-HT reuptake inhibitor, potentiated the stimulatory effects of 5-HT on GtH secretion. Ketanserin (10 micrograms/g; ip), a 5-HT antagonist, did not modify basal GtH levels; however, ketanserin blocked the 5-HT induced release of GtH. These results indicate that 5-HT has a stimulatory effect on GtH release in goldfish, most likely at the level of the pituitary gland.