To investigate the relationship between platelet monoamine oxidase activity and serum levels of testosterone and estradiol in schizophrenic patient, 78 chronic patients were compared with 135 normal controls. 1) The monoamine oxidase activity of chronic schizophrenic group was lower than normal group. 2) In normal group, there was no sex difference in monoamine oxidase activity. But in chronic schizophrenic group, the monoamine oxidase activity in male was lower than female. 3) In male schizophrenic group, the serum estradiol level was lower than normal group. 4) In female schizophrenic group, the serum testosterone level was lower than normal group. The significances of relationship between platelet monoamine oxidase activity and sex hormones are discussed.