BACKGROUND: There is a considerable commercial market, especially within the sports community, claiming the need for antioxidant supplementation. One argument for antioxidant supplementation in sports is that physical exercise is associated with increased reactive oxygen and nitrogen species (RONS) production, which may cause cell damage. However, RONS production may also activate redox sensitive signaling pathways and transcription factors, which subsequently may promote training adaptation. PURPOSE: Our aim was to investigate the effects of combined vitamin C and E supplementation to healthy individuals on different measures of exercise performance after endurance training. METHODS:: Using a double-blinded placebo-controlled design, moderately trained young men received either oral supplementation with vitamins C and E (n=11) or placebo (n=10) before and during 12 weeks of supervised, strenuous bicycle exercise training of a frequency of 5 days/week. Muscle biopsies were obtained before and after training. RESULTS:: After the training period, maximal oxygen consumption, maximal power output, and workload at lactate threshold all increased markedly (P < .01) in both groups. Also, glycogen concentration, citrate synthase (CS), and beta-hydroxyacyl-CoA dehydrogenase (beta-HAD) activity in muscle were significantly higher in response to training (P < .01) in both groups. However, there were no differences between the two groups with regard to any of the physiological and metabolic variables measured. CONCLUSION: Our results suggest that administration of vitamins C and E to individuals with no prior vitamin deficiencies has no effect on physical adaptations to strenuous endurance training.