General practice depressives were treated for 6 weeks with amitriptyline or placebo in a controlled trial. Overall, drug was found strongly superior to placebo. Interactions were examined between drug effects and a number of variables, principally reflecting demographic characteristics, history of illness, severity of illness, and endogenous depression separately in symptoms and stress. Only in the area of severity were significant interactions found. Amitriptyline was superior to placebo in probable or definite major depression on the Research Diagnostic Criteria, but not in minor depression. It was also superior to placebo in subjects with initial scores on the Hamilton Depression Scale of 13-15, and 16 or more, but not with lower scores. Findings indicate that tricyclic antidepressants are of considerable benefit in relatively mild depressions, except in the mildest range.