Summary A study was made of platelet monoamine oxidase (MAO) activity in non-medicated, newly-admitted schizophrenics and institutionalized chronic schizophrenics both on and off medication. These patients were compared to two control groups: normal subjects and brain-damaged institutionalized patients. No relationship was found between platelet MAO activity and the severity or duration of illness, duration of psychotropic medication, presence of auditory hallucinations or institutionalization. Mean platelet MAO activity did not differ significantly between the schizophrenic subgroups and control groups. Thirty-one patients studied before and after treatment with phenothiazines showed no significant change in platelet MAO activity. The findings did not indicate a relationship between schizophrenia, its treatment or outcome and platelet MAO activity.