The electrical resistance of rabbit salivary main duct epithelium has been measured. A small axial electrode, which passed current and measured potential simultaneously, was placed inside the ductal lumen. A cylindrical spiral was wound around the main duct and served as outside current electrode. The instantaneous current voltage relations were linearly up to current densities of 1.5 mA/cm2, independently of the Cl concentration in the bathing solutions. Strong polarization effects were observed in low Cl solutions. There was a significant inverse correlation between the spontaneous potential difference across the epithelium and the epithelial resistance in solutions with either high or low Cl concentrations. In high Cl solutions the epithelial resistance was 12.2 + 1.8 (n = 7) omegacm2. The resistance increased when the mucosal Na and Cl concentrations decreased. After addition of ouabain the resistance always decreased. The temperature dependence of the resistance was determined, and apparent activation energies were calculated. Values for activation energies ranged from 3.2 to 6.5 kcal/mol, depending on the ionic composition of the bathing solutions. Addition of amiloride to the mucosal solution led to an increase in resistance by a factor of 2.1 in high Cl solutions and of 4.1 in low Cl solutions. When ouabain was applied before amiloride, there was no effect on the resistance in high Cl solutions and a smaller increase in the resistance in low Cl solutions. The results of this study support the conclusion that the low resistance of main duct epithelium resides in the cell membranes and is not due to a paracellular pathway.