Controversy still exists regarding the role of serotonin in the regulation of ACTH secretion. We gave normal men 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-dependent stimulatory effect of fenfluramine on both ACTH and cortisol levels. After the highest dose of fenfluramine, mean ACTH and cortisol levels increased from 20.8 pg/ml and 7.3 micrograms/dl to 35.5 pg/ml and 15.1 micrograms/dl, respectively. In a separate study, normal men were pretreated with cyproheptadine, a serotonin antagonist, before the administration of fenfluramine. Cyproheptadine did not significantly alter basal ACTH or cortisol levels, but it did blunt the responses of both hormones to fenfluramine. Cyproheptadine pretreatment did not alter plasma levels of fenfluramine. These findings support a stimulatory role for serotonin in the regulation of ACTH secretion in man.