Ten-week-old jack pines Pinus banksiana Lamb. were exposed to SO2 concentrations from 3·9 to 39 μmol m−3 (0·1 to 1·0 ppm) for durations of 0 to 96 h. Leaf conductance, xylem tension and fructose level decreased, and sulphur level increased, as SO2 concentration and duration increased. Few significant changes were detected before visible injury occurred. Stomatal response was not rapid, and complete closure was found only at high concentrations and long durations. Xylem tension lagged behind leaf conductance in response, but the pattern was very similar. Changes in fructose levels were small and gradual. Sulphur level increased significantly with SO2 exposure after 10 h of fumigation. Multiple regressions, used to describe three-dimensional response surfaces for each variable, each included a crucial interaction term. Although the regression were highly significant, their predictive power was limited due to large amounts of variation in the seedlings.