The effects of unilateral adrenalectomy on the contralateral adrenal gland in rats form the basis of the present communication. The project has been undertaken in an attempt to establish on ultrastructural grounds the stage at which an animal is stabilized on a solitary adrenal gland. The work is chiefly based on electron microscopy, supplemented by gross morphological and light microscopic investigations. Morphological changes in the adrenal gland after removal of the contralateral one were confined exclusively to the cortex and did not involve the medulla. The changes in the fine structure involved most of the organelles that are known to play vital roles in the biosyntheseis of corticosteroid hormones, namely, mitochondria, smooth endoplasmic reticulum (SER) and lipid droplets. The ultra-structural alterations showed remarkably wide variations at different time periods after the operation. The organelles eventually reverted to normal appearances after the 2nd postoperative week. These results have been correlated with the observations of previous workers, and the significance of the present findings, particularly in relation to the functional activity of the adrenal cortex, has been discussed.