The effects of a chronic treatment with ACTH and dexamethasone on the zona glomerulosa of intact rat adrenals were investigated by morphometric methods and EM. ACTH increases the volume of the zona glomerulosa of the cells, nuclei, mitochondrial compartment as well as the surface of SER [smooth endoplasmic reticulum] and mitochondrial cristae. Also noticeable was the hypertrophy of the Golgi apparatus. Opposite effects were obtained after dexamethasone treatment. ACTH is involved in the maintenance and stimulation of the growth and steroidogenic capacity of the adrenal zona glomerulosa.