Native stands of giant sequoia (Sequoiadendron giganteum Bucholz) are being exposed to relatively high concentrations of atmospheric ozone produced in urban and agricultural areas upwind. The expected change in environmental conditions over the next 100 years is likely to be unprecedented in the life span (about 2,500 years) of giant sequoia. We determined changes in physiological responses of three age classes of giant sequoia (current-year, 12-, and 125-year-old) to differing concentrations of ozone, and assessed age-related differences in sensitivity to pollutants by examining physiological changes (gas exchange, water use efficiency) across the life span of giant sequoia (current-year, 2-, 5-, 20-, 125-, and > 2,000-year-old trees). The CO(2) exchange rate (CER) was greater in current-year (12.1 micro mol CO(2) m(-2) s(-1)) and 2-year-old seedlings (4.8 micro mol CO(2) m(-2) s(-1)) than in all older trees (3.0 micro mol CO(2) m(-2) s(-1), averaged across the four older age classes). Dark respiration was highest for current-year seedlings (-6.5 +/- 0.7 micro mol CO(2) m(-2) s(-1)) and was increased twofold in symptomatic individuals exposed to elevated ozone concentrations. Stomatal conductance (g(s)) was greater in current-year (355 mmol H(2)O m(-2) s(-1)) and 2-year-old seedlings (200 mmol H(2)O m(-2) s(-1)) than in all older trees (50 mmol H(2)O m(-2) s(-1)), indicating that the ozone concentration in substomatal cavities is higher in young seedlings than in trees. Significant changes in water use efficiency, as indicated by C(i)/C(a), occurred in trees between ages 5 and 20 years. We conclude that giant sequoias seedlings are sensitive to atmospheric ozone until they are about 5 years old. Low conductance, high water use efficiency, and compact mesophyll all contribute to a natural ozone tolerance, or defense, or both, in foliage of older trees.