The outpatient clinic provides a "well state" patient population for genetic and pharmacologic studies of persons with unipolar (major depressive) and bipolar (manic-depressive) disorders. Such a "well state" population is crucial in identifying biochemical, pharmacologic, or physiologic abnormalities which are present in persons vulnerable to affective disorder, and which may serve as genetic markers to affective illness. Studies of lithium transport suggested that the erythrocyte-plasma lithium ratio is not a marker of vulnerability to bipolar illness.