Fluoxetine, a specific serotonin reuptake inhibitor, was compared to amitriptyline in the treatment of 51 outpatients with primary major depressive disorder. After a 1-week placebo washout, patients were randomly assigned to 5 weeks of treatment with fluoxetine or amitriptyline. Fluoxetine was found to have a therapeutic effect comparable to that of amitriptyline; however, the fluoxetine treatment group had a better Efficacy Index-Side Effects rating and a lower incidence of anticholinergic autonomic side effects. Four amitriptyline-treated patients had to discontinue the study because of serious side effects, while in the fluoxetine treatment group there were no terminations due to side effects. The amitriptyline-treated patients gained significantly more weight than the fluoxetine-treated patients.