- Inadequate response to at least one antidepressant drug - defined as a <50% reduction in severity of depression for a duration of >6 weeks ( > than 8 weeks for fluoxetine), determined by the Massachusetts General Hospital Antidepressant Response Questionnaire. Patients having a score of 18 or more on the 17-item HAM-D will be eligible to participate in the study. The primary efficacy endpoint will be mean change in MADRS score, and the secondary endpoint will be mean change in the HAM-D score