We located a group of healthy young males (aged $20{sim}30$) who had been taking a high dose (more than 5 g) of vitamin C daily for more than one year. We observed that this vitamin C group had plasma levels of vitamin C that were more than three times that of the control group. The control group had not taken any additional vitamin C except for that included in their diets. But the vitamin C group showed significantly lower amounts of Cu/ZnSOD, catalase and glutathione-s-transferase and lower activities of glutathione peroxidase and glutathione reductase in erythrocyte lysates than the control group. However, there was no difference in the plasma levels of lipid peroxides between the two groups. These results suggest that vitamin C offsets its own contribution to anti-oxidant activity by repressing the expression of anti-oxidant enzymes and also excludes the possibility that vitamin C acts as a pro-oxidant in vivo.