In an open study dl-phenylalanine in doses from 75-200 mg/day was administered to 20 depressed patients for 20 days. Patients were classified according to the International Classification of Diseases (ICD). The AMP system, the Hamilton depression scale and the von Zerssen self rating questionnaire were used for documentation of psychopathological, neurologic and somatic changes. In addition a global clinical impression was agreed upon by experienced psychiatrists. 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 more controlled investigations are warranted.