The fine structural changes accompanying the regeneration of adrenocortical transplants in the rat were studied with the electron microscope. One-half gland autotransplants were made to the dorsal musculature of male Wistar rats, weighing approximately 120 gm. Transplants were recovered after 2, 4, 7, 14, 21 and 60 days of regeneration. During the first week of regeneration there was an increase in the granular endoplasmicreticulum at the expense of the agranular form. The internal structure of the numerous mitochondria was transformed from the normal tubular and vesicular forms of the zonae glomerulosa and fasciculata, respectively, to a lamellar type. The quantities of free ribonucleoprotein particles also were reduced. During this period the viable cortical cells are considered to be more deeply involved in protein synthesis and growth rather than hormone biosynthesis. Following 14 days of regeneration and thereafter, the characteristics of the mal intact adrenal cortex became established. Highly osmiophilic “dark cells,” present in the inner zones of the normal intact adrenal cortex, firstappeared after 14 days of regeneration, and were widely scattered throughout the cortexafter three weeks. After two months of regeneration, the “dark cells” were again concentrated in the inner cortical zones. During this latter period, hormone biosynthesis appears to be the major cell function. Possible structural-functional relationships are discussed.