The distribution of the two types of monoamine oxidase within the human adrenal gland was studied. As demonstrated by histochemical techniques, the zona reticularis of the adrenal cortex contains the type A enzyme and the zonas glomerulosa and fasciculata contain little enzyme activity. Biochemical analysis showed that the adrenal medulla contains both types of monoamine oxidase. It is suggested that the type B enzyme is present in chromaffin cells and the type A enzyme is present in vascular and other neural elements. Pheochromocytoma, a tumor of the adrenal medulla, apparently does not contain the B type enzyme.