This is a study of the effect of epiphyseal avascular necrosis on longitudinal growth at the distal end of the femur of the rat. The findings demonstrate that proliferation of epiphyseal cartilage cells, and hence longitudinal growth, depends on an adequate epiphyseal circulation. Necrosis of the epiphysis causes marked change in the epiphyseal plate. These changes consist in walling off of the growth plate with bone and destruction of the epiphyseal cartilage. In the distal end of the femur of the rat this process appears to be irreversible.