Post-menopausal osteoporosis is known to be related to estrogen deficiency. The purpose of this experimental study is to investigate whether or not estrogen administration can prevent or retard the process of osteoporosis induced by oophorectomy in rats. Fifty-two Sprague-Dawley rats, aged five months, were separated into three groups. Group 1 (4 rats) was control group that had sham operation without oophorectomy. Goup 2 (32 rats) was another control group that had bilateral oophorectomy alone. Group 3 (16 rats) was experimental group that had estradiol administration from the fourth week through the eighth week after bilateral oophorectomy. In group 1, all the rats were killed at the fourth week after sham operation. In group 2 and group 3, four rats in each group were killed weekly until the eighth week after oophorectomy. All the animals were subjected to radiological, histological studies on the tibia, ilium, skull, and the sixth lumbar vertebra. Also, histomorphometric measurements were 1. Osteoporotic changes were evident from the fifth week after oophorectomy and increased with time on the radiologic examination in group 2. In contrast, osteoporotic changes were evident from the second week after oophorectomy and progressed with time on the histological and histomorphometric studies in group 2. 2. After administration of estradiol, osteoporotic changes which include thinning of the cortex, decrease of the amount of trabeculae were partially restored on the radiological, histological studies in group 3. 3. Histomorphometric study revealed that the trabecular area percent (the proportion of total area of the trabeculae from the whole metaphyseal area), decreased progressively through the eight week after oophorectomy in gorup 2. Whereas the trabecular area percents of group 3 were higher than those of group 2, but still lower than initial value before oophorectomy. Based on the above observations, estradiol administration appeared to delay the progression of the osteoporotic changes induced by oophorectomy in rats.