Cells and tissues of human and animal are protected against free radicals by several complex mechanisms including the action of antioxidant enzymes, such as superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and catalase. There had been a few reports that simulated or actual weightlessness induced the decrease in activity of antioxidant enzymes and the increase in lipid peroxidation. The purpose of this study was to observe the time-course variation of antioxidant enzymes activities in rats during 14 days of head-down suspension(HDS) at -45° as a model of simulated weightlessness. During HOS, the hepatic activity of SOD significantly decreased (p<0.05) at 3 day of HOS and then maintained a lower value compared to control horizontal position. GSH-Px activity also significantly decreased (p<0.05) at 3 and 7 day of HOS, thereafter showed a slight increasing trend to control horizontal value. The activity of hepatic catalase increased during HOS and the value at the end of HOS showed significant increase (p<0.05). From these results, there is a possibility that weightlessness induce the increase of oxygen free radicals according to the decrease of some antioxidant enzyme activities. The main scavenger to oxygen free radicals is operated via catalase system in rats during HOS. Therefore, we suggest that it is necessary to administrate the antioxidants for protection of the body against oxygen free radicals during first 1 week after exposure of weightlessness, at least.