The effects of acute oral administration of methylphenidate 40 mg versus dextroamphetamine 30 mg versus matched placebo were compared in 11 patients with primary obsessive-compulsive disorder. Dextroamphetamine but not methylphenidate had a significantly greater antiobsessive-compulsive effect as measured by the Comprehensive Psychiatric Rating Scale--Obsessive-Compulsive Subscale, as compared with placebo. This effect appeared unrelated to their effect on depression although a differential effect of the two psychostimulants on anxiety was observed. Although both these stimulants affect serotonin, the differences noted between dextroamphetamine and methylphenidate suggest that catecholamines may be implicated in the pathophysiology of obsessive-compulsive disorder.