Forty patients with a diagnosis of major depressive disorder were entered in a double-blind study to assess comparative clinical response and pharmacologic parameters of fluvoxamine, a highly selective blocker of serotonin reuptake, and desipramine, a noradrenergic agent. Eighteen patients receiving desipramine and 17 patients receiving fluvoxamine completed the study. Fluvoxamine was comparable to desipramine in its antidepressant efficacy and was better tolerated and caused minimal side effects. There was a direct linear relationship between plasma fluvoxamine levels and clinical response and a nonlinear relationship between plasma desipramine levels and clinical response. The pharmacologic specificity of the two drugs was assessed by determining uptake inhibition of serotonin and norepinephrine. The authors found a positive relationship between Hamilton Rating Scale for Depression scores and norepinephrine uptake inhibition for desipramine but found no such relationship between fluvoxamine and serotonin uptake inhibition. Although there was a clear-cut difference in the quality of pharmacologic specificity and a partial relationship to clinical response, the authors were unable to identify neuropharmacologic factors that would predict either treatment response or selective amelioration of symptomatologies in this patient population.