Individual daily dosages of nortriptyline (NT) can be predicted from administration of a 50-mg or 100-mg single test dose, with a determination of the plasma level 24 hours later. Because the 50-mg or 100-mg test dose used in previous studies may cause unmanageable acute side effects in elderly patients, a 25-mg NT test dose was used to establish a 24-hour plasma level in 18 physically healthy, moderately depressed, geriatric outpatients. Correlations between the 24-hour test dose plasma level and steady state levels were done for maintenance dosages of 50, 75, and 100 mg/day. A nomogram was made from the regression equations to predict the dosage required to achieve a steady state concentration within a 50 to 150 ng/ml range. The importance of the ability to predict NT dosage requirements in geriatric patients is indicated by findings that at daily NT doses of 50 and 100 mg, nearly one-half of subjects had steady state levels below or above 50 or 150 ng/ml, respectively.