The central tenet of the Free Radical Theory of Aging (Harman, 1956) and subsequent refinement (Miquel et al ., 1980) is based on the premise that mitochondrial free radical formation is an underlying cause of degenerative senescence. Skeletal muscle, a highly-oxidative tissue exposed to rapidly changing metabolic demands, may play an important role in this process with marked potential to increase oxidative stress (Bailey et al ., 2004). While it would seem reasonable to expect a more pronounced exercise-induced oxidative stress response in senescent compared to young skeletal muscle, human data are lacking which is surprising given the current guidelines recommending physical activity in the elderly. To test this hypothesis, biopsies were obtained via the Bergström technique (Bergström et al. 1967) from the vastus lateralis of 6 yYoung subjects aged 26 (mean) ± 6 (SD) yrs and 6 Aged (71 ± 6 yrs) males who had been matched for body mass, stature, dietary and physical activity status (sedentary). Lidocaine (0.5%) was used as the local anaesthetic (ring block). Samples were obtained at rest and immediately following an acute bout of single-leg knee extensor exercise consisting of 2min at 50% followed by 3min at 100% of the previously determined maximal work rate in a randomised-balanced fashion. Samples were flash frozen and stored at -196°C. Electron paramagnetic resonance spectroscopy revealed a greater resting and exercise-induced increase in the mitochondrial ubisemiquinone radical [Aged: +0.106 ± 0.071 vs. Young: + 0.063 ± 0.051 arbitrary units (AU)/g total protein (TP)] and spin-trapped α -phenyl-tert -butylnitrone adducts that simulation experiments identified were a combination of lipid-derived alkoxyl-alkyl radicals (+215 ± 67 vs. +81 ± 39 AU/mg tissue mass) in the Aged (P < 0.05 vs. Young). Lipid hydroperoxides were also elevated at rest with a more pronounced increase observed during exercise (+0.377 ± 0.342 vs. +0.173 ± 0.234 nmol/mgTP) despite a more marked depletion of ascorbate and uptake of the antioxidants α/, β-carotene, retinol and lycopene in the Aged (P < 0.05 vs. Young). The impact of senescence was especially apparent when oxidative stress biomarkers were expressed relative to the age-related decline in mitochondrial volume density and absolute power output at maximal exercise. The current findings provide the first direct evidence to suggest that human senescent skeletal muscle is indeed exposed to greater resting and exercise-induced oxidative stress and as a consequence is more vulnerable to free radical-mediated lipid peroxidation.