In a previous paper the authors described the morphologic observations that the concomitant administration of ACTH and flavin‐adenine dinucleotide (FAD) to hypophysectomized rats exerted a more potent preventive effect on atrophy of the adrenal cortex of the animals than the single administration of ACTH. The present study was made to electron‐microscopically observe the effect of concomitant administration of ACTH and FAD on atrophy of the adrenal cortex induced with the administration of dexamethasone (Dx). The zona fasciculata of the adrenal gland of rats treated with Dx+ACTH+FAD was morphologically closer in cell organelles such as smooth‐surfaced endoplasmic reticulum, mitochondria and chylomicrons to that of control animals than the counterpart of animals treated with Dx+ACTH only. The zona fasciculata of the adrenal cortex of animals treated with Dx+FAD was morphologically similar to that of animals treated with Dx only. These findings suggested that FAD would potentiate the adrenocorticotropic action of ACTH through its physiologic a