High-affinity binding of the tricyclic antidepressant drug, 3H-imipramine, has been demonstrated in the brain of various species including man. These specific binding sites have many of the characteristics to be expected for the specific site of action of a drug and appear to be associated with the neuronal uptake mechanism for serotonin. Chronic administration of tricyclic antidepressant drugs or the prolonged application of other antidepressant therapies, such as electroshock and sleep-deprivation, resulted in decreases in the density of 3H-imipramine binding sites. Apparently identical 3H-imipramine binding sites have been found in blood platelets from a variety of species including man. Clinical studies have shown that untreated severely depressed patients have a lower density of binding sites in their platelets than control volunteers. Longitudinal studies of these patients indicate that the density of 3H-imipramine binding sites tends not to change during treatment with tricyclic antidepressant drugs and the subsequent recovery from depression. 3H-imipramine binding in brain and platelets is proposed as a new biological marker in depression and as a useful research tool in biochemical and clinical pharmacological studies in affective disorders.