The electrical events associated with the absorption of D-glucose or L-amino acids in renal proximal tubules were studied in microperfusion experiments on rat kidneys in vivo. Intratubular application of these substrates led concomitantly to: 1) a shift of the transepithelial potential into lumen negative direction, 2) a partial depolarization of the tubular cell membranes and 3) a reduction of the electrical resistance of the brushborder membrane. By means of rapid perfusion experiments it was possible to discern two phases in the potential response to substrate perfusion, a fast initial response which reflects a substrate-induced Na+ ion current from lumen to cell, and a slower secondary response which reflects the relaxation of the intracellular ion and substrate concentrations towards new steady states. A quantitative analysis of the data yielded estimates of 1) the apical (Ra) and basal (Rb) cell membrane resistances and of the shunt resistance, Rs, of rat proximal tubule of approximately Ra = 255 omega cm2, Rb = 92 omega cm2 and Rs = 5 omega cm2 (all referred to the quasi macroscopic surface area of the tubular lumen), 2) the conductance of the Na+ and glucose cotransport pathway and 3) the driving forces acting on the cotransport mechanism in the brushborder membrane. The latter were found to be a) the electrical cell membrane potential of -74 mV, b) the Na+ ion concentration gradient between the tubular lumen (clumNa = 145 mmol/l) and the cytoplasm (ccellNa approximately 33 mmol/l) which corresponds to an additional equivalent potential of 51 mV and c) the substrate concentration gradient, which varies according to the experimental conditions. Moreover the analysis provided a quantitative estimate of the relationship between the substrate-induced changes in transepithelial potential or short circuit current and the actual cotransport current in the brushborder membrane. Based on this analysis it is concluded that the stoichiometry of Na+ and glucose flux coupling in the brushborder membrane of rat proximal tubule is close to 1.0.