(1)'Uptake' of phlorizin by intestinal brush border membrane vesicles is stimulated, much as that of D-glucose, by the simultaneous presence of Naout+ and delta psi much less than 0. However, phlorizin contrary to D-glucose, fulfills all criteria of a non-translocated ligand (i.e., of a fully competitive inhibitor) of the Na+,D-glucose cotransporter. (2) The stoicheiometry of Na+/phlorizin binding is 1, as shown by a Hill coefficient of approx. 1 in the Naout+-dependence of phlorizin binding. (3) The preferred order of binding at delta psi much less than 0 is Na+ first, phlorizin second. (4) The velocity of association of phlorizin to the cotransporter, but not the velocity of its dissociation therefrom, responds to delta psi. These observations, while agreeing with the effect of delta psi much less than 0 on the Kd of phlorizin binding in the steady-state time range, also confirm that the mobile part of the cotransporter bears a negative charge of 1. (5) A model is proposed describing the Na+, delta psi-dependent interaction of phlorizin with the cotransporter and agreeing with a more general model of Na+, D-glucose cotransport. (6) The kon, koff and Kd constants of phlorizin interaction with the Na+,D-glucose cotransporter are smaller in the kidney than in the small-intestinal brush border membrane, which results in a number of quantitative differences in the overall behaviour of the two systems.