A study of the sensitivity to fractionation of the growing tibia of rabbits and rats was conducted by comparing the growth of the treated right bone to that of the untreated left side in each individual animal using radiographic measurements. The experimental endpoint was the percentage of normal growth 24 weeks after irradiation in rabbits and 14 weeks after treatment in rats. The results show clear dose-response relationships in all experimental arms. A clear-cut fractionation effect was demonstrated in both species. The alpha/beta-ratios determined by maximum likelihood analysis according to the LQ-model with graded responses were 3.2 Gy (95% C.I. 1.1; 5.6) in rabbits and 6.9 Gy (5.3; 8.7) in rats, when all data were included in the calculations. When single-dose data were excluded the alpha/beta-values were -0.6 Gy (-3.1; 2.3) in rabbits and 5.0 Gy (3.5; 7.0) in rats. Our data provide further evidence that low doses per fraction should be used when irradiation of the epiphysis cannot be avoided in pediatric patients.