The purpose of this study is to evaluate an antioxidative effect of estrogen on the bone in oophorectomized rats. Thirty Sprague-Daley rats were equally divided into 3 groups; group 1 as control group with sham operation, group 2 as experimental group with oophorectomy, and group 3 as oophorectomized group treated with estrogen. Estradiol (5mg/kg BW) was administered three times per week from first to sixth week after oophorectomy. Left tibia was obtained to measure the amount of protein carbonyls as an index of oxidative stress and the activity of antioxidant enzymes. The results were as follows: trahecular bone area in proximal tihia decreased after oophorectomy, which increased in response to estrogen administration. The level of protein carbonylation in hone was not significantly different among all groups. Activity of antioxidant enzymes such ais glutathione reductase(GR), glutathione peroxidase(GP) and glutathione transferase(GST) in bone was not significantly different among all groups. However, the activity of catalase in bone markedly increased in group 3 compared with that in group 1 and group 2. In summary, bone trabecular area increased after admin- istration of estrogen. And estrogen induced the activitv of catalase, which might contrihute to prevent the oxidative damage. However, the glutathione utilizing enzymes such as GR, GP and GST were not significantly affected by estrogen status.