It has been reported that supplementation with the antioxidant vitamins C and E prevents the adaptive increases in mitochondrial biogenesis and GLUT4 expression induced by endurance exercise. We re-evaluated the effects of these antioxidants on the adaptive responses of rat skeletal muscle to swimming in a short term study, 9 days of vitamins C and E with exercise during the last 3 days, and a longer term study, 8 wk of antioxidant vitamins with exercise during the last 3 wk. The rats in the antioxidant groups were given 750 mg/kg body wt. vitamin C and 150 mg/kg body wt. vitamin E per day. In rats killed immediately after exercise, plasma TBAR's were elevated in the control rats but not in the antioxidant supplemented rats, providing evidence for an antioxidant effect. In rats killed 18 hr after exercise, there were large increases in insulin responsiveness of glucose transport in epitrochlearis muscles mediated by a ~2-fold increase in GLUT4 expression in both the short-term and long-term treatment groups. The protein levels of a number of mitochondrial marker enzymes were also increased about 2-fold. Superoxide dismutases (SOD) 1 and 2 were increased about 2-fold in triceps muscle after 3 days of exercise, but only SOD2 was increased after 3 wk of exercise. There were no differences in the magnitudes of any of these adaptive responses between the control and antioxidant groups. These results show that very large doses of antioxidant vitamins do not prevent the exercise-induced adaptive responses of muscle mitochondria, GLUT4 and insulin action to exercise, and have no effect on the level of these proteins in sedentary rats.