[Objectives] Research on oxidative stress in skeletal muscle under disuse conditions is a topic of increasing concern among physiologists. In view of the important role of elevated oxidative stress in the development of disuse muscle atrophy, we hypothesized that hibernators and non-hibernators may exhibit differences in oxidative stress levels and antioxidant defense regulation, thus constituting intrinsic differences in the mechanism of muscle atrophy. Moreover, because of the variations in their energy metabolism and function, differences should also exist in the degree of atrophy and level of oxidative stress in different skeletal muscles. For this reason, we investigated the degree of atrophy in the SOL (slow-twitch) and EDL (fast-twitch) muscles in parallel with the level of oxidative stress, antioxidant defense and underlying mechanism in non-hibernators (Rats, Rattus norvegicus) and hibernators (Daurian Ground Squirrels, Spermophilus dauricus) between artificial and natural disuse. [Methods] We examined changes in muscle wet weight and muscle-to-body weight ratio by electronic balance. Immunofluorescence staining was performed to measure the muscle fiber cross-sectional area (CSA) and fiber type composition. Using a hydrogen peroxide (H2O2) assay kit and malondialdehyde (MDA) assay kit, we measured the concentrations of H2O2 and MDA (level of oxidative stress) in muscle samples. The antioxidant activity was ascertained using specific superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) assay kits. We detected protein expression of the Nrf2/Keap1 signaling pathway and downstream antioxidant enzymes (SOD1, SOD2, CAT, and GPx) in hindlimb loading (HU) Rats and hibernating Daurian Ground Squirrels. Differences in body weight between HU and control Rats and between hibernating inactive (HIB) and summer active (SA) Ground Squirrels were evaluated with an independent samples t-test. Muscle wet weight, ratio of muscle wet weight/body weight, fiber-type distribution, fiber CSA, and all the protein expression levels were analyzed using two-way ANOVA (main effects of muscle type and disuse treatment) followed by independent samples t-test. [Results] Compared to the control Rats, muscle atrophy indices (muscle mass, muscle-to-body mass ratio and CSA) were significantly increased in the SOL and EDL after 14 d of HU. In the Daurian Ground Squirrels, however, no significant changes in these indices were observed in the muscles between the hibernation inactive and summer active groups (Fig. 1 and 2). After 14 d of HU, the proportion of fast type fiber (MyHC II) increased significantly by 206.0% (P < 0.001) in the SOL compared with the control Rats; however, there was no significant change in the proportion of fiber type in the EDL. In Daurian Ground Squirrels, the proportion of slow type fiber (MyHC I) and MyHC II showed no significant changes in the SOL or EDL between the hibernation inactive and summer active groups (Fig. 2). In regard to oxidative stress, H2O2 content increased significantly in the SOL and EDL and MDA content increased significantly in the SOL of HU rats; however, neither were increased in the hibernating Daurian Ground Squirrels, although H2O2 and MDA content decreased significantly in the EDL (Fig. 3). SOD2, CAT, and GPx1 expression markedly decreased in the SOL, as did GPx1 in the EDL, whereas SOD1 expression increased significantly in the SOL of HU rats. In hibernating Daurian Ground Squirrels, however, expression was maintained or increased in both muscles; for example, CAT and GPx1 protein levels increased significantly in the SOL and EDL of the hibernation group (Fig. 5 and 6). Furthermore, compared with control rats, Nrf2, p-Nrf2, and Keap1 protein levels in the SOL of the HU group demonstrated no changes, whereas the expression of Nrf2 and Keap1 in the EDL of the HU group decreased significantly, whereas Nrf2 and p-Nrf2 protein levels in the SOL and EDL of the hibernating Daurian Ground Squirrels increased markedly (Fig. 7). [Conclusion] These results indicate that the Nrf2/Keap1 antioxidant pathway was not activated and downstream antioxidant enzymes were down-regulated in Rats under muscle disuse conditions, which may be an important mechanism in oxidative stress (i.e., increased reactive oxygen species) and induced skeletal muscle atrophy. Conversely, activation of the Nrf2/Keap1 signaling pathway and increase in antioxidant enzymes observed in the Daurian Ground Squirrels during hibernation undoubtedly helped prevent oxidative stress and muscle atrophy. In conclusion, oxidative stress and Nrf2-mediated antioxidant defense appear to differ in skeletal muscles of non-hibernators (Rats) and hibernators (Daurian Ground Squirrels) under disuse conditions, which may be important mechanisms for the differing degrees of muscle atrophy.