The effects of aging and sex on the tensile properties of the rabbit medial collateral ligament (MCL) were examined. The femur-MCL-tibia complex (FMTC) of male and female rabbits, of representative ages before and after epiphyseal closure up to the onset of senescence (i.e., 3.5, 6, 12 and 36 months of age) were examined. A group of 48-month-old female rabbits was also tested. The specimens were tensile tested to failure in order to obtain both the load-elongation curve of the FMTC (structural properties) and the stress-strain curve of the MCL substance (mechanical properties). Significant increases in the linear stiffness, ultimate load and energy absorbed at failure of the FMTC were noted in both the males and females during skeletal maturation, but the FMTCs of the older rabbits began to show a slight decrease in these properties. The ultimate load of the FMTC for the male rabbits reached its plateau at approximately 6 months of age, while that of the females did not plateau until 12 months of age. The modes of failure correlated well with closure of the epiphyses in both sexes, i.e., tibial avulsion failure for the skeletally immature groups and mid-substance for the skeletally mature groups. The modulus of the MCL substance increased during maturation for both sexes until 12 months, and then gradually declined until 48 months. The tensile strength remained relatively constant after 12 months of age but was slightly reduced at 48 months for the females. Thus, the rate of skeletal maturation contributed in part to the differences in the tensile properties of both the male and female rabbits.