To evaluate serotonergic (5-hydroxytryptamine) function in obsessive-compulsive disorder, behavioral and neuroendocrine responses to m-chlorophenylpiperazine (m-CPP; 0.5 mg/kg orally) and fenfluramine hydrochloride (60 mg orally) were examined in 20 patients and 10 healthy controls under double-blind, placebo-controlled conditions. Following m-CPP, but not fenfluramine or placebo, 55% (11/20) of the patients with obsessive-compulsive disorder experienced a transient exacerbation of obsessive-compulsive disorder. Prolactin response was blunted in patients following m-CPP but not following fenfluramine. Patients with greater behavioral response to m-CPP had smaller prolactin responses. Cortisol response to m-CPP and fenfluramine did not significantly differ between the groups. Behavioral and neuroendocrine responses appeared divergent. This does not suggest simply upregulation or downregulation of 5-hydroxytryptamine receptors, but rather complex mechanisms involving multiple neurotransmitter and neuromodulator systems.