Electrophysiologic and tracer experiments have shown that Cl- enters Necturus proximal tubule cells from the tubule lumen by a process coupled to the flow of Na+, and that Cl- entry is electrically silent. The mechanism of Cl- exit from the cell across the basolateral membrane has not been directly studied. To evaluate the importance of the movement of Cl- ions across the basolateral membrane, the relative conductance of Cl- to K+ was determined by a new method. Single-barrel ion-selective microelectrodes were used to measure intracellular Cl- and K+ as a function of basolateral membrane PD as it varied normally from tubule to tubule. Basolateral membrane Cl- conductance was about 10% of K+ conductance by this method. A second approach was to voltage clamp the basolateral PD to 20 mV above and below the spontaneous PD, while sensing intracellular Cl- activity with the second barrel of a double-barrel microelectrode. An axial wire electrode in the tubule lumen was used to pass current across the tubular wall and thereby vary the basolateral membrane PD. Cell Cl- activity was virtually unaffected by the PD changes. We conclude that Cl- leaves Necturus proximal tubule cells by a neutral mechanism, possibly coupled to the efflux of Na+ or K+.