During an asymptomatic phase platelet MAO activity was examined in 28 unipolar patients, 8 of whom previously had developed mania during clomipramine treatment for depression. The platelet MAO activity was not lower in these switchers than in 20 unipolar patients who did not become manic during a similar clomipramine treatment. High but nonsignificant correlations were found among platelet MAO activity and age of onset of mania, duration of clomipramine treatment before the switch, and the duration of manic behavior. These clinical variables correlated significantly amongst themselves. The high and normal platelet MAO activities of these patients neither support nor rule out a Bipolar I diagnosis. This study supports the pharmacologic basic of the induction of mania during clomipramine treatment.