The relationship between plasma levels of clomipramine and desmethylclomipramine and clinical response was studied in a group of patients with obsessive-compulsive neurosis. In general the response was best in the middle range of plasma concentrations, showing the inverted U curve well recognized with nortriptyline. However, the results of this study suggest that the response of compulsive rituals correlates with levels of plasma clomipramine, while depression appears to correlate with plasma levels of desmethylclomipramine. The former relationship may be the stronger. There may also be some relationship between side effects and relative plasma levels of clomipramine and desmethylclomipramine.