A linear two-compartment open-ended model was used to describe the kinetics of strontium in the rat as a function of age. Strontium-85 was used as the tracer in the eight age groups (39 to 389 days) studied. The numerical solution of the parameters, compartment sizes, and transfer rates was performed by the Berman (NIH-OMR-SAAM) program on an IBM-7094 digital computer. The fit of the experimental data, plasma, urine, feces, and whole-body retention to the model was unique and consistent. The parameters measured, with the exception of the urinary excretion rate, decreased as a function of age in the growing animal. The accretion rate was found to be linearly related to the rate of increase of body weight and to the total exchangeable calcium compartment. This study also demonstrated that the same model could be used, with the same degree of reliability, in man and the rat, which suggests that there is a qualitative similarity in the skeletal dynamics of both animals. The advantage of this digital computer analysis is that it affords a quantitative measure of the accuracy of the calculated parameters as well as objectivity and reproducibility.