Murphy et al. were the first who reported on reduced platelet monoamine oxidase (MAO) activity in schizophrenic patients. After this report followed several investigations of the platelet MAO activity in schizophrenia, some of which were contradictory with Murphy et al. In this study, we first chose fourteen chronic schizophrenic subjects (4 males and 10 females) among the hospitalized patients, whose diagnoses were established according to the diagnostic criteria of Feighner et al. and Kolb. The patient platelet MAO activities were investigated twice at intervals of twelve months and compared with the normal volunteers. The volunteers were eleven laboratory personnel (5 males and 6 females) in the first control group, and twelve personnel (6 males and 6 females) in the second control group. Six of the same laboratory personnel (3 males and 3 females) were tested in both contral groups. Platelet MAO activity was determined according to the radiochemical method by Wurtman et al. The details were described elsewhere. MAO activity was expressed in nmoles of metabolic products per mg of platelet protein per 20 min. Statistical comparisons were made with the Mann-Whitney U-test. The two values of patient platelet MAO activity (0.503 in the first determination and 0.543 in the second) were significantly lower than the corresponding control values (0.790 in the first and 0.728 in the second). There was some difference (8% increase in the second) between the two mean values of platelet MAO activity in the schizophrenic group, while no appreciable change of psychotic state was observed in the patients during the twelve month period. The same trend of MAO variation was found in the control group. A tendency for higher platelet MAO activity was found in female subjects both in the normal and in the schizophrenic group. In the second part, repeated determinations of platelet MAO activity was made in an another group of six schizophrenic patients (2 males and 4 females) and one male control over almost a two year period. The platelet MAO activity in all subjects showed some difference over time without relationship to the psychological and psychotic state. Moreover, rather greater changes in platelet MAO activity were found in the acute stadium of schizophrenia with a tendency for increased MAO activity related to antipsychotic medication and in the later return to the premedicated low MAO level following continued medication. In the final part, diurnal changes of platelet MAO activity was examined in one male control, two schizophrenics (1 male and 1 female) and one male epileptic. A small variation with time occurred with reduction of inconsistent tendency. Our results confirmed the reduction of platelet MAO activity in schizophrenic subjects, and also showed the MAO activity changes over time. The platelet MAO reduction possibly suggests the existence of an abnormal monoamine metabolism in the brain. However, the basis for this finding needs to be explored extensively.