In addition to dopamine, serotonin (5-hydroxytryptamine, 5-HT) has been reported to play an important role in schizophrenia. Besides blocking dopamine, atypical antipsychotics also block 5-HT receptors. The clinical efficacy of the atypical antipsychotic clozapine is associated with the 5-HT antagonistic action of the drug and a high serotonergic tone before treatment. The atypical antipsychotic olanzapine has a receptor-binding profile similar to that of clozapine. The present study investigated whether treatment with olanzapine blocks hormone release induced by the 5-HT2c agonist m-chlorophenylpiperazine (m-CPP) and, if so, whether this 5-HT antagonistic effect is related to treatment response. Eighteen male schizophrenic patients participated in this study. All patients were challenged with m-CPP (0.5 mg/kg orally) in a double-blind, randomized, placebo-controlled design after a drug-free period of at least 2 weeks. Adrenocorticotropic hormone (ACTH), cortisol, and prolactin plasma levels were measured every 30 minutes up to 210 minutes after challenge. Patients were treated for 6 weeks with 10 mg olanzapine daily in an open design, after which the challenge tests were repeated. Olanzapine significantly blocked m-CPP-induced ACTH, cortisol, and prolactin release, suggesting that it is a potent 5-HT2c antagonist in vivo. This 5-HT antagonistic effect of olanzapine was not significantly correlated with treatment response. Also, no significant correlation was found between m-CPP-induced hormone release before treatment and clinical response after treatment with olanzapine. These findings suggest that olanzapine is a potent 5-HT2c antagonist in vivo but that this is unrelated to its clinical efficacy in this nonrefractory sample of schizophrenic patients.