There are several possible mechanisms of the antioxidant effects of vitamin C on platelets. First, one may consider that increased plasma vitamin C protected against free radical–mediated inactivation of PDNO. In this study, the plasma vitamin C level was increased from 28 (baseline value) to 107 μmol/l at 2 h. However, a recent in vitro study by Jackson et al. (26) showed that impaired endothelium-derived NO release by superoxide was not prevented by vitamin C at concentrations <150 μmol/l. Accordingly, this possibility is unlikely. Second, it is possible that the protective effects of vitamin C on PDNO release are mediated by the antioxidant property of vitamin C at the intraplatelet level. Jackson et al. (26) demonstrated that vitamin C was effective in competing with NO for superoxide at concentrations ≥1 mmol/l. Thus, they speculated that the vasorelaxing effects of exogenously administered vitamin C on the constricted vascular strips by superoxide were due to the intracellular increases in vitamin C, although they did not measure intracellular vitamin C concentrations. In this study, we measured intraplatelet and plasma vitamin C concentrations in smokers at 2 h after vitamin C administration, which were 8.3 mmol/l and 107 μmol/l, respectively. Thus, our findings indicate that intraplatelet vitamin C concentrations are higher in platelets than in plasma and are sufficient enough to scavenge superoxide anion. Third, the antioxidative effects of vitamin C may have prevented oxidation of intraplatelet GSH, resulting in the preservation of GSH levels. However, vitamin C administration did not change intraplatelet GSH levels in nonsmokers and smokers. The process of platelet aggregation involves the generation of oxygen free radicals (27), and intracellular antioxidants such as vitamin C and GSH may be consumed during platelet aggregation. Accordingly, we calculated consumption of intracellular vitamin C and GSH during platelet aggregation, and the consumption rates were similar between the two groups before vitamin C administration. After vitamin C administration, consumption of intraplatelet vitamin C was significantly increased in both nonsmokers and smokers, and, again, no difference was found between the two groups. However, consumption of intraplatelet GSH after vitamin C administration was significantly lower in smokers than in nonsmokers. These findings suggest that in smokers, vitamin C may have preserved intraplatelet GSH during platelet aggregation.