We carried out animal experiment in order to determine the effect of resection of nerves, muscles and tendons on the induction of osteoporosis. One hundred and forty rats, weighing approximately 250 gm, were divided into seven groups. In Group I, as a control group, the three skin incisions, one on the anterior aspect of the knee, one on the anteromedial aspect of proximal thigh, and one on the posterior aspect of proximal thigh were made and wounds were sutured. In Group II, the patellar tendon and the extensor digitorum longus tendon were divided to disable knee extension. In Group III, all the knee flexors were divided. In Group IV, all the knee extensors and flexors were divided. In Group V, the femoral nerve was divided to disable knee extension. In Group VI, the sciatic and obturator nerves were divided to diable knee flexion. In Group VII, the sciatic, and obtrator nerves were all divided. Before the procedures, bone density was measured by photon absorptiometry and postoperatively, the measurement was repeated every two weeks until the eighth week. At the time of each measurement, two rats were sacrificed out of each group and were submitted to microphotographic measurement of the tibial cortices. The results were as follows. l. In the muscle and tendon resection groups (Group II, Group III, Group IV), the bone density significantly decreased two weeks after operation and then, gradually increased four, six and eight weeks. 2. In the nerve resection groups (Group V, Group VI, and Group VII), the bone density decreased significantly two and four weeks after operation and then gradually increased six, and eight weeks after operation. 3. In both motor resection and denervation, the paralysis of both flexors and extensors of the knee, caused more decrease in bone density than the paralysis of either flexor or extensor. 4. In the muscle and tendon resection groups, the bone density was significantly more decrease two weeks after operation when the flexors were resected, but thereafter there was no significant difference between the flexors and extensors resection. In the nerve resection group, the paralysis of knee flexors caused more decrease in bone density than the paralysis of knee extensors throughout the entire experimental period. 5. The nerve resection groups demonstrated significantly more severe loss of bone density than muscle and tendon resection groups throughout the entire experimental period. 6. The values of bone density measured by photon absorptiometry correlated well with the cortical thickness indices. Bssed upon these observations, we came to the following conclusions. 1. The osteoporosis following injuries of the nerve, muscle, and tendon is most significant at two weeks after injury. 2. Denervation causes more severe osteoporosis than muscle and tendon injuries. 3. It is suggested that the photon absorptiometry is a reliable means in the evaluation of the osteoporosis.