The present study examined the effect of long-term, moderate physical exercise on trabecular bone volume (TBV), calcium content, 3H-proline uptake, and the activities of alkaline and acid phosphatases in lumbar vertebrae of aging and senescent mice. It became apparent that if physical activity starts at an early stage of life, i.e., prior to middle age and is extended until old age, it exerts beneficial effects on trabecular bone mass and mineralization. Such a positive effect is not obtained if the training program is initiated after middle age. The training-induced reduction in bone loss was accompanied by a significant decrease in acid phosphatase activity whereas no changes took place with regard to the activity of alkaline phosphatase. Long-term physical exercise also enhanced the uptake of 3H-proline by lining cells along the bone trabecules. In spite of its moderate nature, the endured training program served as a stress factor for the involved animals, a fact that was manifested by an increase in the serum levels of corticosterone. Thus, it seems that whereas young animals respond favorably to such a stimulatory stress, older animals lose this ability of adaptation.