Serum levels of amitriptyline plus nortriptyline were measured by radioimmunoassay at 1 and 6 weeks in depressed out-patients treated with amitriptyline for 6 weeks. Serum concentrations at 6 weeks were higher in older patients. Serum levels showed no relationship to clinical response at 6 weeks, and a week inverse relationship with response at 2 weeks. Routine monitoring of serum levels appears to be of little value in depressed out-patients treated with amitriptyline.