Steady-state plasma tricyclic antidepressant levels were determined in 65 patients undergoing treatment for depression with either amitriptyline hydrochloride or nortriptyline hydrochloride to determine if common factors such as age, race, sex, or smoking status were predictors of steady-state drug levels that have been shown to vary up to 36-fold. Evaluation of these factors did not disclose differences in the rate of demethylation of amitriptyline to nortriptyline, or steady-state tricyclic levels in the amitriptyline-treated patients. No differences were found in the nortriptyline-treated patients except regarding race. Black patients had significantly higher (50%) nortriptyline plasma levels than did white patients, which may explain the more rapid response to tricyclic treatment demonstrated in blacks. Decreased rates of nortriptyline metabolism in blacks can result in increased side effects and treatment failure if the therapeutic plasma range is exceeded.