This study was performed to observe the influence of moderate treadmill running on bone of middle-aged male rats. Seventy 15-month-old Wistar rats were used. Ten initial controls (IC) were killed on day 0. Among the 60 others, three groups of ten exercised rats (E) run 1 h/day, 6 days/week at 60% of their maximum aerobic capacity. On days 30, 60 and 90 of the training period, 20 rats, ten E and ten R (resting animals), were killed. Femoral failure stress never varied and was never different in E and R during the experiment. On day 90 whole body mineral content and mineral density were higher in E than R. Simultaneously, total, diaphyseal and metaphyseal femoral densities were lower in R than IC or than in E. No difference was observed between IC and E. In resting rats, urinary deoxypyridinoline excretion (a marker of bone resorption) increased between days 0 and 90, while it did not change in runners. These results indicate that in middle-aged rats, moderate running prevents decrease in bone mineral density, probably by inhibiting bone resorption.