The stoichiometric properties of the renal outer cortical brush-border membrane D-glucose transporter are studied. Experiments which establish the glucose/sodium, glucose/phlorizin and phlorizin/sodium stoichiometries are reported. Three independent method of determining the substrate/activator (glucose/sodium) stoichiometry for coupled transport systems are presented and discussed. One of these, the "Static Head Method," is introduced here for the first time. This type of experiment appears to be more generally applicable than the usual procedure of directly measuring the coupled fluxes of substrate and activator to determine stoichiometric coupling ratios. The results presented in this paper demonstrate that the glucose/sodium/phlorizin stoichiometry of the renal outer cortical brush-border membrane D-glucose transport system is 1:1:1.