Plasma dopamine-beta-hydroxylase (DBH) activity was studied in two different populations of chronic schizophrenic patients and assayed by two independent laboratories. No significant difference between schizophrenic patients and normal controls was found although in both groups chronic undifferentiated schizophrenics with paranoid features had a trend towards lower DBH activity than the other patients and controls. In addition, DBH and monoamine oxidase (MAO) activities were studied in 13 schizophrenic patients and available first degree-relatives. There was no association of low MAO and low DBH activities within the schizophrenic families.