To study the possible implication of endogenous serotonin in the control of glucagon secretion in man, normal volunteers were subjected to alpha-cell stimulation before and after oral treatment with serotonin antagonists (cyproheptadine and methysergide) and with an inhibitor of serotonin synthesis (para-chlorophenylalanine, PCPA). After administration of cyproheptadine (16 mg daily, for two days) the glucagon responses to arginine (N=12) and to insulin-induced hypoglycemia (N=9) were more marked than in the control experiments (differences between maximal elevations: +165 pg/ml, P less than 0.0001, and +197 pg/ml, P less than 0.02, respectively). After methysergide treatment (9 mg daily, for two days), a potentiation of arginine-provoked glucagon secretion was also observed (+260 pg/ml, P less than 0.002; N=7). Similarly, after PCPA administration (2 g daily, for four days) the alpha-cell responsiveness to both aminogenic (N=12) and hypoglycemic (N=7) stimuli was enhanced (+108 pg/ml, P less than 0.05, and +164 pg/ml, P less than 0.05, respectively). Since glucagon secretion is potentiated by treatment with drugs which either antagonize serotonin action or inhibit its synthesis, the suggestion can be made that endogenous serotonin modulates alpha-cell function in man by acting as an inhibitor.