Human normal and neoplastic adrenocortical cells were incubated under stimulation with ACTH and observed by scanning electron microscopy. Cultured normal adrenocortical cells gathered into small clusters, each cell of which had a polarity or orientation evidenced by two different aspects. In one aspect, the cell surface rounded up and microvilli protruded vertically. Pits were found among or close to the groups of microvilli. In the other aspect, the cell surface was flattened and well-developed microvilli ran horizontally. These two aspects of the cultured cells were thought to correspond to the cell surface facing the intercellular space and that facing the perisinusoidal space, respectively. In incubated cell clusters of adrenocortical adenomas with Conn's syndrome, most cells lost this polarity or orientation and unstimulated cells existed as unit of the clusters, but all adenoma cells reacted to ACTH in the same manner. Microvilli were distributed unevenly. Filopodia were noticed in some cells. Bleb-like structures appeared frequently and some of them were about to be extricated from the cell surface as in normal adrenocortical cells. Adrenocortical adenomas with Cushing's syndrome showed remarkable responses to ACTH. Their cell surface was unclean with the adherence of fragmented cytoplasm and bleb-like structures. Horizontally running elongated microvilli were almost indistinguishable from collagen fibrils. Moreover, collagen fibrils were entangled with microvilli.