Sixty-four elderly depressed outpatients were treated with nortriptyline for seven weeks. Plasma nortriptyline and its main metabolite, 10-hydroxynortriptyline, were measured weekly. No relationship was found between levels of 10-hydroxynortriptyline and clinical response. Plasma levels of the trans isomer, E-10-hydroxynortriptyline, were significantly lower when dizziness and symptoms of orthostatic hypotension were reported, although there was no significant correlation with actual orthostatic drop in systolic pressure. Plasma level of 10-hydroxynortriptyline was not significantly correlated with the other reported side effects.