Populations of Geranium carolinianum, winter annual plant common in disturbed habitats vary in their folair response to sulfur dioxide and pollution resistance is characteristic of populations sampled from areas in which SO2 has been a prominent stress. The physiological basis of this ecotypic response was investigated using a whole-plant gaseous exchange system in which leaf resistance to H2O efflux and SO2 influx were concurrently monitored. Individual plants of distinct SO2 susceptibility were exposed to pollutant concentrations of either 0.4, 0.6, or 0.8 microliter/liter in both the dark and light. Total SO2 flux to the plant which is the sum of leaf adsorptive and absorptive loss, varied as an inverse function of leaf resistance and the relationship was modeled using linear regression techniques. Total SO2 flux was partitioned to leaf surface and internal fractions using estimation procedures with the regression analysis. SO2 flux into the leaf interior, the pollutant fraction responsible for causing foliar injury, was strikingly similar for resistant and sensitive plants at each concentration. Resistant plants must absorb 30 percent more SO2 than their sensitive counterparts in order to exhibit comparable levels of foliar injury. Therefore, in G. carolinianum the predominant explanation for genetically controlled and quantitatively inherited differences in plant response to SO2 is not variable pollutant flux but rather disparate physiological-biochemical processes affecting pollutant toxicity, cellular perturbation and repair.