Loss of bone mass with aging and consequent increase in fracture incidence among the elderly is a major medical and societal problem. There is currently no accepted treatment for type II, or age-related (senile) osteoporosis. Physical exercise has been suggested as a possible treatment for or preventive measure against age-related bone loss, but most past studies have been compromised by lack of sufficient controls. Also, several important issues regarding the interaction between exercise treatment and other factors have been inadequately studied. In this investigation, the Beagle dog, which has been shown to lose bone with aging in a manner similar to that of humans, will be used as a model to study the effects of increased physical exercise on bone mass in a controlled environment. 24 female Beagles in a laboratory colony ranging in age from 8 to 13 years, equivalent to humans in the sixth through eighth decades, will be subjected to 20 minutes per day of graded treadmill running for 8 months. The effects of the treatment will be evaluated in vivo using dual energy photon absorptiometry or quantitative digital radiography, and computed tomography. Using these techniques, bone mineral content of the distal and midshaft femur, lumbar vertebrae, and mandible-maxillae, and cortical geometry of the midshaft femur will be determined 12 months prior to treatment, at the beginning and end of treatment, and 8 months after treatment. Individual pre-treatment baseline rates of change in these parameters will be compared with rates of change during treatment and post- treatment periods. Activity levels of the animals in all periods will also be monitored. The long-term goal of the study is to determine if moderately strenuous physical exercise can be used to treat or prevent age- related bone loss. The influence of several specific factors on the efficacy of exercise treatment will be investigated: initial subject age and relative activity level or baseline increases in mechanical loading, and response to deconditioning following an exercise treatment period.