In a large (n = 459) sample of adults free of psychiatric, neurologic, and endocrinologic disease, platelet monoamine oxidase activity was analyzed by multiple regression of the demographic variables age, race, and gender on enzyme activity. Reported here are variations for all three demographic variables such that significantly greater enzyme activity is seen in female, older, and white subjects relative to male, younger, and black subjects. For each demographic group the data demonstrated a curvilinear relationship of age and enzyme activity with a nadir of activity at age 30. For this sample enzyme activity nearly doubled between subjects at age 30 and at age 80. We believe this study to be the first to report racial differences in this enzyme activity and to analyze normative data for this enzyme by multiple regression techniques.