Basal prolactin concentrations were measured before treatment in 18 children and adolescents with obsessive-compulsive disorder as well as in 15 of these patients after 4 and 8 weeks of clomipramine treatment. Basal prolactin levels were influenced by a history of chronic tic disorder and by the duration and severity of obsessive-compulsive symptoms. Clomipramine administration significantly increased basal prolactin levels. A slight decline in prolactin levels during the last 4 weeks of clomipramine treatment was positively correlated with a favorable treatment response and negatively correlated with duration of illness. If the changes in prolactin levels observed during clomipramine treatment are due primarily to changes in serotonergic neurotransmission, these data suggest that clomipramine treatment of obsessive-compulsive disorder produces an adaptive decrease in the responsiveness of serotonergic receptors.