Effects of oxidative stress (OS) on the fetus were studied under ischemic induced by clamping the uterine vessels of pregnant rats. OS was evaluated by superoxide (O-.2) released from red blood cells. Fetal RBC-O-.2 of the ischemic rats (clamping for 10 min and 20 min) was significantly higher than that of the control group (p < 0.01). Maternal RBC-O-.2 tended to increase, but not significantly. When readings before and after clamping were compared, the activity of fetal RBC SOD had increased in the ischemic group (clamping time for 10 min), but significantly decreased in the 20 min clamping rats (p < 0.01). Maternal SOD activity tended to decrease along with clamping time. The amount of lipidperoxide was significantly greater in the fetal ischemic liver subjected to clamping for 20 minutes than in the tissue subjected to clamping for 10 minutes. The lipidperoxide level was significantly higher in the fetal ischemic brain exposed for 10 minutes than in the control. These results indicate that fetal RBC-O-.2 increased along with ischemic time, and the maternal RBC-O-.2 had been influenced by the increase in fetal RBC-O-.2. Based on these observations, it was concluded that the OS due to ischemic injury had a more adverse effect on the fetal brain than on the liver.