Study patients will receive, double-blind, either intravenous or oral clomipramine on two consecutive days: 150 mg on Day 1 and 200 mg on Day 2, administered in the Stanford General Clinical Research Center (inpatient unit), with continuous monitoring of vital signs. On the fourth day after completing the pulse loading, the patient will come to the Psychiatry outpatient clinic to begin 12 weeks of oral clomipramine, starting at 200 mg/day and increasing rapidly, as tolerated, to 250 mg/day. To be eligible, patients must be aged 18-55, in good physical health, able to discontinue all medications except a benzodiazepine if necessary, have OCD of moderate or greater severity for at least one year, no psychotic disorder, no substance or alcohol abuse/dependence diagnosis within the previous six months, no history of bipolar disorder, no tic disorder or Tourette's syndrome, and no personality disorder severe enough to interfere with cooperation with the study procedures. They must also have failed to benefit markedly from at least two adequate (>= 8 weeks) trial of serotonin reuptake inhibitors: citalopram 20 mg/day: clomipramine 200 mg/day; fluoxetine 40 mg/day; fluvoxamine 200 mg/day; paroxetine 40 mg/day; setraline 100 mg/day.