Computing techniques are used to simulate the course of uptake of K(+), Na(+), and Cl(-) by low-salt roots. Measurements of the fluxes of these ions in high-salt roots are used to calculate membrane permeabilities, which are then used to calculate cell uptake. In this way it is possible to test the predictive value of different models for the location of sites of salt uptake in the cell.On the basis of the permeability data used, it is suggested that anion transport at the plasmalemma must be larger than that provided by Mechanism I and that there is a need for transport of both cations and anions at the tonoplast. To account for the observation that the level of salt at equilibrium in the tissue is very nearly independent of concentration, it is suggested that there must be some feedback from vacuolar content to rate of transport at the tonoplast.