Platelet 3H-imipramine binding was studied in 37 patients fulfilling Research Diagnostic Criteria for major depressive disorder, examined before and after four weeks of treatment with minaprine 200 mg/day, and in 19 healthy controls. Mean baseline Bmax values of depressed patients were found to be significantly lower than those of controls, while no significant difference between the two groups was observed with respect to mean Kd. Treatment with minaprine did not significantly affect Bmax or Kd in depressed patients. When patients who responded to treatment (n = 18) were compared with nonresponders (n = 19), mean baseline Bmax values were found to be significantly lower in the former group, whereas mean Kd values did not differ. It is hypothesized that reduced 3H-imipramine binding may represent a predictor of a favorable response to antidepressant drugs which potentiate serotonergic transmission.