Monoamine oxidase (MAO) activities were measured in platelets from insulin-dependent and non-insulin-dependent diabetic subjects and in platelets from nondiabetic controls. Circulating levels of glycosylated hemoglobin (HbA1) were determined simultaneously. Mean MAO activities were not significantly different in any of these groups. MAO activity did not relate to the age of the individual, but mean values for females were higher than mean values for males in healthy controls and in insulin-dependent diabetics. In this study mean HbA1 levels were higher in female than in male diabetics. There was no relationship between MAO activity and HbA, level when results for males and females were analyzed separately.