In an open study dl-phenylalanine in doses from 75--200 mg/day was administered to 20 depressed patients for 20 days. At the end of the trial 12 patients (8 with complete, 4 with good response) could be discharged without any further treatment. 4 patients with partially untypical depressions experienced mild to moderate responses, whereas 4 patients did not respond at all to the phenylalanine administration. Depressive "core symptoms" as depressed mood, retardation and/or agitation were preferentially, anxiety and sleep disturbances moderately and hypochondriasis and compulsiveness were not influenced. It is concluded that dl-phenylalanine might have substantial antidepressant properties and that further controlled investigations are justified.