Data from several previous studies link clomipramine's potent serotonergic effects to its clinical efficacy in reducing the symptoms of obsessive-compulsive disorder (OCD). To investigate this relationship further, we administered the serotonin (5-HT) receptor antagonist, metergoline, and placebo to ten patients with OCD in a crossover study carried out under double-blind, random-assignment conditions. In a previous study of untreated patients with OCD, we found no differences in the behavioral response to single-dose administration of metergoline or placebo. In the present study, patients with OCD receiving clomipramine hydrochloride on a long-term basis (with an average 40% lessening in OC symptoms) responded to a four-day period of administration of metergoline with significantly greater self- and observer-rated anxiety compared with the four-day placebo period. Obsessive-compulsive symptoms also tended to be greater during the metergoline phase, with significant drug-time interactions for both OC symptoms and anxiety peaking on day 4 of the metergoline phase. As anticipated, metergoline lowered plasma prolactin concentrations (providing evidence of physiologically significant 5-HT antagonism) but did not alter plasma clomipramine concentrations. These data further support the hypothesis that clomipramine's therapeutic behavioral effects in OCD are mediated via serotonergic mechanisms.