The effects of repeated exposure to ethylene oxide on lipid peroxidation and glutathione metabolism in both rat liver and brain were examined. Increased levels of malondialdehyde (MDA) in the liver were observed after 6 and 13 weeks of exposure to ethylene oxide. The increased level of MDA observed in the hepatic homogenates of the treated rats reflected that of the microsomal fraction. On the other hand, no change in the level of MDA was detected in the brain of rats either at a 6- or 13-week treatment. Glutathione reductase activity was found to decrease at 6 or 13 weeks in liver and brain of treated rats. Both reduced and oxidized forms of glutathione in homogenates of liver and brain obtained from treated rats were, however, similar to those of the control at 40 h after the last exposure in individual experiments. To elucidate the cause of lipid peroxidation, the time course of glutathione content after exposure with ethylene oxide were studied in more detail. Significant decreases in both GSH and GSSG content in these organs were detected shortly after exposure to ethylene oxide and their levels recovered gradually with time and reached the control values at 40 h in the liver, although the changes were less significant in the brain as compared with those in the liver. These results suggest that enhancement of lipid peroxidation in the microsomal fraction of the liver after repeated exposure to ethylene oxide may possibly arise from repeated depletions of glutathione to certain critical levels and less removal of lipid peroxidation.