Hood and Dare cultivars of soybean, Glycine max (L.) Merr., vary in their foliar response to ozone. The physiological basis of this variation was investigated as a function of leaf age through an analysis of ozone flux data, leaf developmental morphology, and analogue modeling techniques. At all concentrations and exposure times, resistance to 03 flux in the gas phase of the diffusive pathway (i.e., boundary layer and stomate) did not account fully for variation in pollutant uptake rates into the leaf interior. Ozone molecules experienced a residual resistance to diffusion that is not shared by effluxing water vapor molecules.