The purpose of this study was to develop a simple osteopenia model under reduced physical activity. Both hindlimb of 7 weeks old male rats was immobilized for 10 days. Rats were sacrificed either directly after immobilization or after 20 days of free remobilization. BMC and BMD of the femur and tibia were measured in the whole bone and in 7 regions divided along the long axis of the epiphysis from distal to proximal. After immobilization, BMC and BMD of the whole tibia were lower than those of the age-matched control animals (-17.5 and -7.2%, respectively). Similar trends were observed in the femur. Immobilization-induced decreases in BMC and BMD were observed in each of the 7 divided regions. The rate of decrease in BMC and BMD was region-dependent and was pronounced in the epiphysis and the metaphysis compared to the diaphysis. Our immobilization model has potential for evaluating the therapeutic interventions to prevent osteopenia caused by inactivity.