Purpose: Antioxidant supplementation may modulate systemic cortisol and interleukin-6 (IL-6) responses to prolonged exercise, but it is unclear whether such effects are also associated with a reduction in the magnitude of immunodepression. The purpose of the present study was to examine the effects of daily vitamin C (L-ascorbic acid, 1000 mgIdj1) and vitamin E (RRR-alpha-tocopherol, 400 IUIdj1) supplementation on immunoendocrine responses to prolonged exercise. Methods: Twenty healthy, recreationally active males cycled for 2.5 h at approximately 60% of maximal oxygen uptake after 4 wk of placebo (PLA, N = 10) or antioxidant (AO, N = 10) supplementation. Results: A significant group x time interaction was observed for plasma cortisol concentration (P = 0.008), and the postexercise increase was greater (P G 0.05) in the PLA compared with AO group(approximately 170% compared with an approximately 120% increase above baseline). Plasma IL-6 concentration was significantly increased after exercise to a similar extent in both groups. Plasma free F2-isoprostane concentration was significantly increased after exercise and was unaffected by AO supplementation, whereas plasma TBARS was unaffected by exercise in the PLA group but was lower after exercise in the AO group than in the PLA group. Circulating neutrophil count was significantly increased after exercise, and in vitro bacteria-stimulated elastase release per neutrophil was significantly decreased to a similar extent in both groups. Conclusions: These results suggest that 4 wk of AO supplementation may blunt the cortisol response to a single 2.5-h bout of prolonged exercise independently of changes in oxidative stress or plasma IL-6 concentration, but it is not effective at modulating the exercise-induced neutrophilia or depression of neutrophil function.