Hypertrophy of the adrenal cortex of the rat following prolonged administration of ACTH, unilateral adrenalectomy, castration and pregnancy was associated with narrowing of the glomerulosa and outward displacement and increased cellularity of the subglomerular zone. Only late in pregnancy did the glomerulosa become wider than usual. The main enlargement occurred in the fasciculata and reticularis. The alterations in the inner zones of the gland differed in each case and they might have been produced by the specific effects of either ACTH or gonadotrophins.