Isolated intestinal segments from rats or hamsters were recirculated with balanced salt solutions containing fluorocarbon emulsion to provide 6 vpc oxygen. The lumen contained an axial Ag-AgCl electrode, and the serosal surface was surrounded by a cylindrical shell of Ag-AgCl. Transmural impedances were measured at frequencies from 0.01-30 kHz before and after removal of the mucosal epithelium. The resistance of intercellular junctions, RJ, the distributed resistance of the lateral spaces, RL, and the distributed membrane capacitance, CM, were computed from the relations between frequency and impedance. Activation of Na-coupled solute transport by addition of glucose, 3-0-methyl glucose, alanine or leucine caused two- to threefold decreases of transepithelial impedance. Typical changes induced by glucose in hamster small intestine were RJ 30----13 omega, RL 23----10 omega, and CM 8----20 microF (per cm length of segment). Half maximal response occurred at a glucose concentration of 2-3 mM. The area per unit path length of the junctions (Ap/delta chi = specific resistance divided by RJ) in glucose activated epithelium was 3.7 cm in hamster midgut and 6.8 cm in rat. These values are close to the 4.3 cm estimated independently from coefficients of solvent drag and hydrodynamic conductance in glucose-activated rat intestine in vivo. The transepithelial impedance response to Na-coupled solute transport was reversibly dependent upon oxygen tension. It is proposed that activation of Na-coupled solute transport triggers contraction of circumferential actomyosin fibers in the terminal web of the microvillar cytoskeletal system, thereby pulling apart junctions and allowing paracellular absorption of nutrients by solvent drag as described in the previous accompanying paper. Anatomical evidence in support of this hypothesis is presented in the following second accompanying paper.