Peroxiredoxins (Prx) are a family of thioredoxin or glutathione dependent peroxidases. Six isoforms have been described (Prx1-6). We investigated Prx 1-6 protein expression in the M. vastus lateralis of trained (Ex) and sedentary (CON) wild-type mice (WT_Ex: n=6, WT_Con: n=7) and transgenic mice with a heterozygous deficiency of the manganese superoxide dismutase (MnSOD_Ex: n=11, MnSOD_Con: n=9). Mice performed a running exercise for 60 min./d at 15 m/min and an inclination of 5U for 5 days a week (duration: 8 weeks). In WT_Con and MnSOD_Con, Prx 1 and 2 expression was similar, whereas Prx 3- 6 proteins were significantly down-regulated in MnSOD_Con compared to WT_Con. In WT_EX, Prx 3-6 proteins were significantly decreased after 8 weeks running exercise. Prx 1 and 2 protein was unchanged. In MnSOD mice, physical exercise further decreased Prx3-protein but increased Prx 1 and 4. Conclusions: Physical activity induces alterations of Prx 1-6. The selective regulation may indicate that other antioxidative systems, which have not been investigated in the present study, are regulated as well. The differences in the regulation of the Prx- isoforms may be dependent from the basal oxidative status.