The pharmacological probe, meta-chlorophenylpiperazine (m-CPP), administered orally to patients with obsessive-compulsive disorder (OCD) has been shown to induce an acute exacerbation in OCD symptoms as well as an exaggerated anxiogenic response in comparison with controls. The mechanism of m-CPP's behavioral effects in humans remains controversial. To further study m-CPP's actions in OCD patients, we completed a series of double-blind pharmacological challenges in 12 OCD patients. Six OCD patients received four separate challenges: placebo, metergoline, m-CPP, and metergoline plus m-CPP; the second group (n = 6) received metergoline and metergoline plus m-CPP in separate challenges. OCD patients receiving placebo or metergoline alone failed to show evidence of significant changes on any of the behavioral rating scales, in contrast to the patients who received m-CPP alone who exhibited significant increases in anxiety and OCD symptoms. However, the 12 OCD patients who received pretreatment with metergoline before m-CPP experienced no significant changes from baseline OCD symptoms or other behavioral changes. m-CPP's ability to elicit elevations in plasma prolactin was blocked by metergoline pretreatment. Metergoline's ability to block m-CPP's effects on behavior and plasma prolactin lends further support to a serotonergic mediation of m-CPP's effects, including its elicitation of OCD symptoms.