Ultrastructural differences between normal adrenal medulla and pheochromocytoma were studied. Medullary cells exhibit almost the same appearance revealing cytoplasm filled with numerous catecholamine granules. However, the ultrastructures of pheochromocytomas are quite variable, being composed of two major cell types: Type I with catecholamine granules and Type II without them. Type I cells are divided into three sub-groups according to their cytoplasmic catecholamine granules and vacuoles, which can be seen in the same section. This fact can be interpreted as the expression of cellular activities. The co-existence of these cell types in pheochromocytoma would be one of the ultrastructural evidences of the catecholamine metabolism disorders. In order to compare the catecholamine granules, their sizes were measured, calculating their mean diameter in each cell type. Significant differences were shown on their mean value and distribution curve between normal medullary cells and each cell type of pheochromocytoma. These differences were discussed in relation to the catecholamine abnormalities in pheochromocytoma.