Hormonal responses to oral paroxetine were examined in a group of healthy subjects. The calcium response to serotonin (5-hydroxytryptamine, 5HT), mediated by platelet 5HT2A, was also measured. Paroxetine elicited a cortisol response that was directly correlated with the magnitude of platelet calcium response. The cortisol response was also correlated with the trait of impulsivity. These results suggest that paroxetine may be a useful probe in studies of serotonergic systems.