The effects of progressively higher CO₂ levels on the foliar antioxidant status were studied by growing soybean (Glycine max Merrill cv. Cresir) plants at decreasing distances from natural CO₂ sources of geothermal origin in central Italy. When compared with neighbouring controls grown under normal CO₂ concentration (C), soybean leaves grown at 2 × C, 7 × C and more than 20 × C showed a substantial reduction in the size of ascorbate pool and in the activity of Cu,Zn-superoxide dismutase; both the content of ascorbic acid and the activity of ascorbate peroxidase declined at 2 × C and 7 × C and recovered to the control values at 20 × C. The foliar titre of glutathione disulfide and the activities of glutathione disulfide reductase and Mn-superoxide dismutase progressively increased as CO₂ concentration increased in ambient air. The results obtained suggest that the immanent risk of dioxygen toxicity associated with photosynthetic electron flow could be reduced in the presence of high CO₂ levels. On the other hand, depending on both the CO₂ exposure regimes and the cell compartment considered, high CO₂ could promote oxidative processes which cause GSH oxidation and require an enhanced cellular ability to scavenge superoxide anion and hydrogen peroxide.