The effect of aminonucleoside of puromycin on growing bone in the rat is characterized first by stimulation with accelerated maturation of bone and cartilage cells. This change is climaxed by a metaphyseal lesion identical with osteitis fibrosa produced by other means. Cellular growth activity practically ceases thereafter. The appearance of excess osteoid after the microscopically determined advent of renal lesions suggests a relationship between the two. After growth ceases, anorexia, thyroid atrophy, and hyperplasia, first of the adrenal glands and finally of the parathyroid glands, make further assessment of the pathogenesis of the bone lesions extremely difficult.