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Aim: We examined the changes in oxidative stress, mitochondrial function and muscle atrophy during aging in mice., Methods: We used 6-, 12- and 24-month (6 M, 12 M and 24 M)-old C57BL/6J mice. Skeletal muscles were removed from the lower limb and used for quantitative real-time polymerase chain reaction, immunoblotting and histological analyses., Results: The muscle weight and myocyte cross-sectional area were significantly decreased in the 12 M and 24 M mice compared with those of the 6 M mice. The levels of the oxidative stress markers, nicotinamide adenine dinucleotide phosphate oxidase 2, nicotinamide adenine dinucleotide phosphate oxidase 4, mitochondrial 4-hydroxy-2-nonenal and 3-nitrotyrosine, were significantly higher in the 24 M mice compared with those of the 6 M mice. Furthermore, the 24 M mice had lower levels of mitochondrial markers, peroxisome proliferator-activated receptor gamma coactivator 1 (PGC)-α, peroxisome proliferator-activated receptor gamma coactivator-1β, sirtuin-1, adenosine triphosphate synthase mitochondria F1 complex α subunit 1 and mitochondrial cytochrome c oxidase 1. The ubiquitin-proteasome pathway genes muscle ring finger-1 and atrogin-1 were significantly upregulated in the 12 M and 24 M mice, and protein synthesis markers (phosphorylated-Akt and -p70 ribosomal S6 kinase) were significantly lower in the 24 M mice compared with the 6 M mice (all P < 0.05)., Conclusions: These findings have important implications for the mechanisms that underlie sarcopenia and frailty processes. Geriatr Gerontol Int 2020; 20: 78-84., (© 2019 Japan Geriatrics Society.)