With a variable strain-rate testing machine, we loaded the proximal tibial epiphyses of male and female rats twenty-five to sixty-five days old. We measured tibial lengths, epiphyseal-plate cross-sectional areas, loads at failure, and displacements at failure in 560 tibiae, and calculated total energy absorption, nominal shear stress, and maximum tension (bending) stress for each epiphyseal plate. In fifty-six tibiae, we used subfailure loads to locate the initiation of cartilage failure. Increasing loads were required to cause failure at increasing but physiological rates of loading, demonstrating a viscoelastic property of the cartilage. The strength of cartilage from female rats was usually greater than that from age-matched males. There was a decrease in strength at pubescence. The mean values of maximum bending stress and nominal shear stress decreased significantly for females at forty-five days old and for males at fifty days. On histological examination of tibiae loaded to 50 per cent of failure energy, internal cracks within the plates were seen which were in planes developing the highest shear stresses. These cracks preceded the disruption of the posterior fibers that were in maximum tension, but usually were included in the eventual path of the failure crack.