A mathematical relationship is developed which relates bone density to the daily bone stress history resulting from physical activity. The model postulates that bone density is distributed to maintain a constant level of daily cumulative stimulus for all bone tissue. The daily loading history is expressed in terms of the peak cyclic stress or strain energy density magnitudes and the number of daily loading cycles. An empirical weighting factor in the model, m, accounts for possible differences in the relative importance of the cyclic stress magnitude and number of daily loading cycles. The method which is presented can be used to study the relationship between bone morphology and daily functional loading. It is also shown that bone density can be directly related to the daily history of external forces imposed on the body in bones subjected to a regular pattern of loading. Bone density in the calcaneus is formulated in terms of the daily history of ground reaction forces. This model is applied to two running studies in which calcaneal density had been measured. Estimates of the stress exponent, m, determined from a best fit of the model predictions to these studies indicate that bone density is found to be far more sensitive to the magnitude of the load than to the number of daily loading cycles. Using the best estimate for the value of m, calcaneal densities are predicted for a broad spectrum of activity lifestyles. The influence of supplemental exercise on bone density is also explored. Additional discussions include the application of the model to 1) an analysis of NASA bedrest studies, and 2) the correlation of bone loss to the decline in activity level with age. These results indicate that it may be possible to design exercise programs to achieve desired changes in bone mass.