The results of pilot studies of the activities of platelet monoamine oxidase (MAO) and catechol-0-methyl transferase (COMT) in the blood of selected schizophrenics and the families of schizophrenics is presented.No statistically significant difference was found between the blood COMT levels of 21 control subjects and 26 schizophrenics, whereas the values found for platelet MAO activity were significantly lower for the schizophrenic group than for the control group.In one acutely disturbed first-admission schizophrenic the platelet MAO activity increased to a normal level in parallel with the clinical improvement, whereas in the relapsing schizophrenics the platelet MAO activity remained at its initial level although the clinical picture improved. No consistent findings with regard to the platelet MAO activity emerged from the study of 3 families having a history of schizophrenia.