It has been long established by reports of Kiyono (1914), Aschoff (1924), Arndt (1927), and others that the interstitial capillary system of the adrenal cortex is stained vitally with acid dyes such as trypan-blue or lithium carmine, and that thus it belongs to the reticuloendothelial system (RES), but in regard to whether or not the adrenocortical cells, like the endothelium, have an ability of vital staining there is disagreement in opinion even among light microscopists (see Bachman, 1954).In this paper, the adrenal cortex of mice given gold-sol by intravenous injection was observed electronmicroscopically, and at the same time ATP-ase activity of the same organ of controls was also investigated on the level of electronmicroscope.