• To evaluate further the basis for the reduced activity of platelet monoamine oxidase (MAO) found in chronic schizophrenic patients, a number of characteristics of the enzyme were compared between patients and controls. Equivalent and statistically significant reductions in activity of the enzyme were found in the patients when tyramine and benzylamine were used as the substrates in comparison to previously reported reductions with tryptamine as the substrate. Michaelis constants for platelet MAO from chronic schizophrenic patients with reduced enzyme activity were not different from controls. Dialysis of the enzyme from patients and controls yielded no changes in activity. Studies of other platelet enzymes, including succinate dehydrogenase, cytochrome C reductase, and lactate dehydrogenase in patients, normal controls, and a subgroup of normal controls with reduced MAO activity, showed no parallel reductions in activity in patients or controls with low MAO activity. These findings suggest that the reduced MAO activity found in chronic schizophrenic patients is apparently not accounted for by nonspecific changes in platelets or platelet mitochondria.