Background : The purpose of this study was to observe the variation of malondialdehyde (MOA), an indirect index of oxidative damage, following 4-week of head-down suspension (HOS) at -45° in rats as a model of simulated weightlessness. Methods : We also measured the activities of antioxidant enzymes, such as superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and catalase for clarifying the mechanisms of renal oxidative damage. Results : MDA was increased (P<0.05) at the 4th week of HOS rats compared to control horizontal positioned rats. Following HOS, the renal activity of SOD was also significantly increased (P<0.01) at the 4th week of HOS whereas the changes of renal GSH-Px and catalase activities were not significantly different from controls. The expression of renal SOD mRNA used by polymerase-chain reaction method showed the similar pattern with the change of renal SOD activity and was more increased (P<0.05) than control horizontal positioned rat. Conclusion : These results indicate that simulated weightlessness induces the augmented SOD gene expression in the kidney which results in increased SOD activity, and thus increased production of MDA due to increased production of hydrogen peroxide. And under this condition, GSH-Px and catalase do not play their protective roles against hydrogen peroxide.