3H-p-Aminoclonidine binding to platelets of children and adolescents with major depressive disorder was compared to that of a healthy control population. Significantly higher alpha 2-adrenoceptor Kd and Bmax values were observed in the patient population. 3H-Dihydroalprenolol binding to lymphocyte membranes of the same patient population showed significantly higher beta-adrenoceptor Bmax values than controls. Control females had significantly higher beta-adrenoceptor Kd values than control males, and the female patients had significantly lower beta-adrenoceptor Kd values than control females. 3H-Imipramine binding to platelets of these patients showed significantly higher imipramine Kd values in patients with a suicide attempt, whereas the imipramine Bmax values were significantly increased in patients with major depressive disorder with or without a suicide attempt. We propose that increased platelet alpha 2-adrenoceptor Kd and Bmax values, together with increased platelet imipramine Kd values, may serve as possible biological markers for children and adolescents with major depressive disorder and a tendency toward suicide. Elevated platelet imipramine and lymphocyte beta-adrenoceptor Bmax values may be biological markers for juvenile depression, and decreased beta-adrenoceptor Kd values may be a biological marker for depression in young females.