1. The human adrenal cortex has been studied at death by correlation of plasma hydrocortisone levels with histology. It has been proved that the totally lipid depleted adrenal cortex seen at death is not functionally exhausted. 2. The pattern of cellular replacement in the human adrenal cortex has been studied. Cell replacement in the human adrenal cortex takes place only in the compact cells of the zona reticularis and ACTH stimulated zona fasciculata. The centripetal theory of cell growth does not hold for the adult human adrenal cortex. 3. The distribution of mitochondria and Golgi apparatus in the human adrenal cortex has been studied. Mitochondria are plentiful in the compact cell and the glomerulosa cell, but fewer in the clear cell. True Golgi impregnation has not been demonstrated except as a network in the clear ultrastructure which probably represents impregnation of the endoplasmic reticulum. 4. The ultrastructure of the human adrenal cortex has been studied. Three cell types are founds (i) the compact cell in the zona reticularis and ACTH-stimulated fasciculata, characterised by the presence of frequent mitochondria, prominent vesicular endoplasmic reticulum, microvilli and scanty lipid. The vesicular component has been shown by cell fractionation to be endoplasmic reticulum. (ii) the clear cell characterised by fewer mitochondria, less endoplasmic reticulum, absence of microvilli and preminent lipid globules. (iii) the glomerulosa cell characterised by relatively frequent mitochondria, dense endoplasmic reticulum, a variable lipid content and small or absent microvilli. (iv) ACTH-stimulation has the effect of altering the fine structure of the clear cell to approximately that of the compact cell
