Ten patients with DSM-III diagnoses of nonbipolar recurrent major depression were studied in an attempt to assess the relationship between 3H-imipramine binding site density and clinical depressive state. They were compared with eight healthy controls who had no past or family history of affective disorders. Evaluations with the Hamilton Rating Scale for Depression and the Self-Rated Scale for Depression were done on the same day as platelet collection at baseline, and also at 2 and 5 weeks after the beginning of treatment with tricyclic antidepressants. The number (Bmax) and the affinity (Kd) of platelet 3H-imipramine binding sites were highly correlated with the improvement of the clinical depression. These results raise the interesting possibility that a decrease in 3H-imipramine binding sites may be a state-dependent marker in patients suffering from nonbipolar recurrent major depression.