Controversy still exists regarding the role of serotonin in the regulation of prolactin (Prl) and growth hormone (GH) secretion in man. We gave healthy male volunteers three oral doses (0.5, 1.0 and 1.5 mg/kg) of fenfluramine, a serotonin-releasing agent and uptake inhibitor, and a corresponding placebo. There was a significant dose-response effect of fenfluramine on Prl but not on GH levels. Following the highest dose of fenfluramine, mean Prl levels increased from 9.7 ng/ml to 42.3 ng/ml. In a separate study, subjects were pre-treated with cyproheptadine, a serotonin antagonist, before the administration of fenfluramine. Cyproheptadine did not significantly affect basal Prl or GH levels, but it did blunt the response of Prl to fenfluramine. Cyproheptadine pre-treatment did not alter plasma levels of fenfluramine. Our findings support a stimulatory role for serotonin in the regulation of Prl secretion in man. They also suggest that serotonin does not have a major influence on GH secretion in man.