Depressed patients who were suitable for drug treatment were allocated randomly to treatment for four weeks with either amitriptyline in doses reaching 150 mg daily or with L-tryptophan in a maximal dose of 8 G daily. Both in-patients and out-patients were included. The trial was double-blind and ratings were made at the start of treatment and weekly for the subsequent four weeks: the patients were then followed for a further six months. Both groups of patients improved steadily over the course of four weeks and there were no marked differences between the treatment groups though there was some tendency for the improvement of the tryptophan-treated patients to fade between the third and fourth weeks. Within the tryptophan group anxious patients improved least. It is concluded that L-tryptophan probably has some antidepressive action in patients with depressive illness of moderate severity.