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Seedlings of sugar maple (Acer saccharumMarsch.) were exposed for 46 days to 700 ppm of C02, 200 ppb of ozone, and 700 ppm of C02+ 200 ppb of ozone. A significant increase in the activity of H202scavenging enzymes, i.e. ascorbate peroxidase [EC 1.11.1.11] and catalase [EC 1.11.1.6], was measured due to the action of 03. This increase was rather negatively affected by elevated C02. A tendency of decreased activity of glutathione reductase [EC 1.6.4.2] and superoxide dismutase [EC 1.15.1.1] due to the action of 03was detected. Elevated C02does not provide enhanced tolerance to oxidative stress in the seedlings of sugar maple. Changes in the activity of antioxidant enzymes were more pronounced in the young leaves (developed during the experiment) than in the old leaves (developed before starting the experiment).