In the vascular system of the adrenal, the medullary artery is considered to be a shunt which controls cortical function by changing its diameter. The medullary arteries of ACTH-treated and control rats were observed by scanning electron microscopy of methacrylate corrosion casts of blood vessels. After ACTH treatment, the medullary arteries were constricted mostly at their distal segments and sometimes also at other portions. This constriction probably results in an effective augmentation of adrenocortical blood flow. The diameter of capillaries in the glomerular zone and medullary sinusoids was markedly increased by ACTH. Small protrusions on the corrosion casts which supposedly corresponded to capillary fenestrations became prominent after ACTH administration in the fascicular and reticular zones.