To evaluate the possible abnormality in MAO activity in affective disorders, blood platelet samples were obtained from 80 patients with mania and depression. Blood-platelet MAO activity was measured by a newly developed assay procedures using serotonin as substrate. MAO activities in 121 normal adult subjects were in a range of 2.49-12.05 nM/mg protein/hour, with the mean values of 4.91 +/- 1.72 (+/-S.D.) for men and 6.88 +/- 1.99 for women. (p less than 0.001) MAO activities in the manic and depressed patients were in a range of 0.65-13.40 nM/mg protein/hour, and both manic and depressed patients showed the mean value very familiar to that in the normal subjects. Bipolar depressed patients did not exhibited lower MAO activity in the blood platelets than other clinical subtypes of depressive illness, including unipolar, involutional, neurotic and chronic characterological, and first-episode depression. No significant differences were established between these five subcategories of depression, while significant higher values were evident in female than male patients (p less than 0.001). No correlation was found between the MAO activity and serotonin levels in the blood platelets either in the normal subjects or in the depressed patients.