The effect of short-term immobilization on bone mineral density (BMD) of femur was examined in mature adult rats. Both hindlimbs of 8 months old male rats were immobilized for 10 days. Rats were sacrificed either directly after immobilization or after 20 days of free remobilization. BMD was measured in the whole femur and in 7 regions divided along the long axis of the epiphysis from distal to proximal. After immobilization, no significant changes were observed in the whole and regional BMD of the femur. Surprisingly, 20 days after remobilization BMD of the whole and regions was dramatically reduced. These data suggest that short-term immobilization may affect bone deterioration during the recovery period rather than inactivity period, despite restoration of normal activity.