Thirty-two patients who had responded to amitriptyline (150 mg daily) when suffering from a depressive illness were allocated either to receive placebo or to remain on the same medication for one year. Plasma concentrations of the drug were regularly estimated. There was no correlation between plasma concentration and subsequent residual affective morbidity. In spite of considerable encouragement, three of the patients did not take the prescribed amitriptyline and they all relapsed. Five out of sixteen patients who received placebo relapsed. None of the patients who continued to take amitriptyline relapsed. It is emphasized that the patients studied were selected, inasmuch as they were apparent responders to amitriptyline. It is concluded that this group of patients should continue to be treated with antidepressant medication for eight months after apparent recovery, and care should be taken to ensure the patients' compliance.