Photosynthetic stimulation and stomatal conductance (Gs) depression in Quercus ilex leaves at a CO(2) spring suggested no down-regulation. The insensitivity of Gs to a CO(2) increase (from ambient 1500 to 2000 micromol mol(-1)) suggested stomatal acclimation. Both responses are likely adaptations to the special environment of CO(2) springs. At the CO(2)-enriched site, not at the control site, photosynthesis decreased 9% in leaves exposed to 2x ambient O(3) concentrations in branch enclosures, compared to controls in charcoal-filtered air. The stomatal density reduction at high CO(2) was one-third lower than the concomitant Gs reduction, so that the O(3) uptake per single stoma was lower than at ambient CO(2). No significant variation in monoterpene emission was measured. Higher trichome and mesophyll density were recorded at the CO(2)-enriched site, accounting for lower O(3) sensitivity. A long-term exposure to H(2)S, reflected by higher foliar S-content, and CO(2) might depress the antioxidant capacity of leaves close to the vent and increase their O(3) sensitivity.