INVESTIGATIONS of adreno-cortical morphology have been overshadowed by extensive experimental and clinical reports. The results of many such experiments on the adrenal cortex of the rat have been considered by many workers applicable to theories of human physiology. Such an attitude may be fallacious, because specific morphological differences exist in the adrenal cortices of various mammals. Since function can be correlated so often with morphology, a thorough consideration of the histology of the adrenal cortex of the rat may be useful at this time. It is the purpose of this paper to describe the normal morphology of the adrenal cortex of the laboratory rat from birth to senility. Following the classical work of Arnold (1), a vast amount of literature has accumulated on adreno-cortical morphology. Because this literature has been summarized so well in a recent monograph by Bachmann (2), no attempt to review the general literature will be made.