In this study the authors evaluate the relationship existing between reduced glutathione (GSH) and increased indexes of muscle performance. GSH has a protective action on the cell either against the oxidative stress or for its ability of removing through out the body xenobiotic substances circulating. 15 male competitive pentathlon athletes were the sample of this research. The aim of the research is to evaluate the possible role of GSH to determine the anaerobic threshold through its blotting function.