To investigate the inflences of training intensity on bone development in growing rats, young (4-wk-old) Wistar strain were subjected treadmill running 1 h/day, 5 days/wk for 8 wk with a variety speed: 30 m/min (T30 group),40 m/min (T40 group),50 m/min (T50 group). The results be summarized as follows: 1) In the distal femoral epiphysis, longitudinal growth rate of T30 and T40 groups were significantly higher than C (sedentary age-matched controls) and T50 groups, respectively, and the calculated rate of cartilage cell production of T50 group was the lowest among all 4 groups. 2) No difference in bone mineral content (BMC) of femoral mid-shaft was found between any of the groups, but the BMC/body weight was significantly higher in all T groups than C group. Moreover, BMC/body weight increased with the speed of runnning except for T50 group which was suspected to result in converse bending of femur while running, compared with T30 and T40 groups, in connection with the different contraction styles of muscle groups attached on femur. On the basis of these findings, we concluded that physical training may underlie the various effects, such as acceleration, non-influence and/or inhibition, respoding to exercise intensity, on the bone development.