Eighteen depressed outpatients were treated for 6 wk with a mean daily dose of 121 mg of nortriptyline. The mean plasma level was 138 ng/ml during treatment. Therapeutic response was monitored by the Zung Self-Rating Depression Scale and the Hamilton Depression Scale administrered by two psychiatrists blind to the tricyclic used, dose, and plasma levels. Eight patients recovered (Hamilton less than or equal to 6) by the fourth week and 12 by the sixth week. During the steady-state (wk 4 to 6) there was a positive correlation between the weekly Hamilton scores and the weekly nortriptyline levels (p less than 0.01). The 9 patients with mean plasma levels between 50 and 139 ng/ml had a better therapeutic response after 6 wk measured by percent recovered (p less than 0.005). Zung score (p less than 0.05), and Hamilton score (p less than 0.025) than the 9 patients with mean plasma levels between 140 and 260 ng/ml.