In a 6-week, double-blind study involving 34 endogenously depressed patients, plasma trimipramine levels of two dosage groups, 75-mg/day and 150-mg/day, were compared with regard to clinical efficacy as determined by scores on the Hamilton Rating Scale for Depression, the Clinical Global Impressions scale, and the Zung Self-Rating Depression Scale. Both dosage levels of trimipramine produced prompt, consistent, and progressive antidepressant effects. No correlation between plasma levels and clinical efficacy was found.