The effect of therapeutic levels of irradiation on appositional bone growth was compared with its effect on longitudinal growth in the skeletally immature rat model. The widths and lengths of the tibiae and fibulae of young rats were studied at 2, 4, 6, and 12 weeks after exposure to 17.5 Gy x-irradiation to the knee region of the right leg, with and without the aminothiol radioprotectant amifostine 20 minutes before radiation. Irradiation retarded growth in the width of the tibia to a greater extent (19%-27%) than longitudinal growth (9%-21%). The appositional growth discrepancy decreased over time, whereas the length discrepancy increased. The proximal fibula, in contrast, undergoes a normal decrease in width over time, and irradiation retarded this contraction by 14%. Appositional growth does not appear to be spared from the damaging effects of irradiation, but a catch-up phenomenon is observed that is not seen in longitudinal growth. Amifostine reduced the radiation-induced loss in tibial width by 40% to 50% and in length by 12% to 30%.