This study was designed to investigate the influence of endurance training on the activities of the scavenger enzymes superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) measured in erythrocytes from sedentary subjects, amateur bicycle racers and in professional bicycle racers who were cycling in two real cyclist contests including the top-class race Vuelta a España (VCE). Under resting conditions the SOD activity was higher (p less than 0.01) in cyclists than in controls. The activities of CAT and GSH-Px were higher (p less than 0.05 and p less than 0.01, respectively) under resting conditions in professional cyclists versus measured both in sedentary subjects and amateur cyclists. The enzyme activities were not modified significantly in professional cyclists after a bout of exercise of 22 km in 5 hrs, but the SOD activity was increased (p less than 0.05) and CAT activity reduced (p less than 0.05) after 2800 km in 20 days (VCE). The present results shown that aerobic endurance training, when at a professional level, produces an increase in the erythrocyte activities of the main free radicals scavenger enzymes.