The microcirculation of the rat adrenal gland was studied with a corrosion cast technique and a freeze-cracking method under scanning electron microscopy. 1) The adrenal arteries run in the surface of the adrenal gland, branch off into arterioles which finally form the subcapsular plexus. This plexus is a dense anastomosing network covering the surface of the adrenal gland. 2) The cortical capillaries arise from the subcapsular plexus, and run vertically towards the adrenal medulla. These capillaries have sinusoidal structures. 3) The cortical capillaries make up a honey-comb pattern in the outer glomerulosal zone and become dense and thick in the cortico-medillary border. 4) The medullary, cortical and looped arteries penetrate the subcapsular plexus and run vertically into the cortex. 5) Most medullary arteries have 1 to 3 branched which run back towards the surface to join in the subcapsular plexus. Hence the author termed these recurrent arteries. The medullary artery occasionally has a branch which runs to the medulla, but does not anastomose with the cortical capillary plexus. 6) The capillaries of the medullary artery are smaller, and the plexus which is made of these capillaries is more coarse than the cortical plexus. These plexes have a coarse anastomosis with cortical capillaries and then run into the medullary vein. 7) The medullary veins drain into one adrenal vein. The adrenal vein anastomoses with the subcapsular plexus in rats as well. 8) Similar results were observed in the freeze-cracked specimen. In addition, in the endothelium of the cracked cortex, many polymorphous fenestrations were observed which were analogous to liver sinusoidal fenestrations. 9) The shape and size of the fenestrations vary with the technique of making the specimen. The fenestrations were very small and uniform when the adrenal gland was not irrigated.