Previous studies implied association of osteoporosis with estrogen deficiency, immobilization and low calcium absorption only through epidemiological studies. There have been only a few experimental studies verifying the etiologic factors of osteoporosis in vivo condition. Authors conducted an experimental study using white rats(Sprague Dawley) in order to find out what the endocrinological and biochemical changes of experimentally induced osteoporosis are and how they behave with or against each other. White rats, eighty five in number of 3 to 6 months females and weighing 220±12.7gm were divided into four groups. They consisted of Group I(n=10) for control, Group II(n=25) for bilateral oophorectomy, Group III(n=25) for bilateral division of sciatic nerve and hip spica cast immobilization, and Group IV(n=25) for bilateral oophorectomy and bilateral division of sciatic nerve plus hip spica cast immobilization. Blood samples were taken preoperatively and postoperatively at six weeks to check Estradiol and Osteocalcin levels there of. And then, rats were sacrified immediately after the second sampling to retrieve femora for bone mineral density measurement and torsional stress test. Estradiol levels before operation were 21.4±13.3pg/ml for Group I, 31.6±3.1pg/ml for Group II, 25.6±4.5pg/ml for Group III and 33.7±4.5pg/ml for Group IV, respectively. There were no significant differences observable among the groups. Estradiol levels at six weeks postoperatively were 42.3±18.8pg/ml for Group I, 5.4±2.7pg/ml for Group II, 40.8±5pg/ml for Group III and 6.2±2.3 pg/ml for Group IV, respectively. Apparent reductions in group II and IV were proved of statistical significance. Osteocalcin levels preoperatively were 1.2±0.6ng/ml for Group I, 1.7±0.4ng/ml for Group II, 1.5±0.1lng/ml for Group III and 1.5±0.1.lng/ml for Group IV, respectively. At six weeks postoperatively they were 1.6±0.1lng/ml for Group I, 1.7±0.3ng/ml for Group II, 1.8±0.3ng/ml for Group III and 1.2±0.1lng/ml for Group IV, respectively. The differences and changes among the groups and measurements were not of statistical significance. Bone mineral contents at six weeks postoperatively were 0.248±0.03g for Group I, 0.177±0.03g for Group II, 0.226±0.04g for Group III and 0.092±0.01g in Group IV, respectively. Low values of Group II and IV compared to those of Group I and III were of statistical significance.(P=0.0001) Torsional strength of bones at six weeks postoperatively were 4.0±0.2N/m for Group I, 1.5±0.1N/m for Group II, 1.5±0.1N/m for Group III and 1.4±0.1N/m for Group IV, respectively. Decreases of experimental groups(II, III, IV) compared to that of control group(I) were of statistical significance, but differences among the experimental groups were not of significance(p>0.05). For above observations, it was possible to conclude that osteoporosis measurable by bone mineral content and torsional stress test was caused by oophorectomy and immobilization in vivo, the former of which was more rapid and profound than the latter during early phase. When both factors, i.e., oophorectomy and immobilization are exerated simultaneously. the resultant osteoporosis was found in higher degree than either factor only, but not at incremental degree as one may expect.