We have developed a novel approach to study K channels in the lateral membrane of principal cells (PC) in rat cortical collecting ducts (CCD). The technique consists of 1) exposing the CCD apical membrane, 2) removing the intercalated cells adjoining a PC by gentle suction through a pipette, and 3) applying patch-clamp technique to the lateral membrane of PC. Functional viability of the PC was confirmed by three indexes: 1) maintenance of physiological cell membrane potentials (-85 ± 3 mV); 2) depolarization of the cell membrane potential with 1 mM Ba 2+ ; and 3) hyperpolarization of the cell potential with 0.1 mM amiloride. Two types of K channels were identified: a low-conductance K channel and an intermediate-conductance K channel. In cell-attached patches the slope conductance of the low- conductance K channel was 27 pS and that of the intermediate-conductance K channel was 45 pS. The open probability (P(o)) of the 27-pS K channel was 0.81 ± 0.02 and was not voltage dependent. In contrast, the P(o) of the 45- pS K channel was 0.23 ± 0.01 at the spontaneous cell membrane potential and was increased by hyperpolarization. In addition, decrease of the bath pH from 7.4 to 6.7 reduced the 27-pS K channel current amplitude in a voltage- dependent manner, but the P(o) was not affected. Finally, two time constants were required to fit open- and closed-time histograms of both populations of K channels. Application of 1 mM Ba 2+ completely blocked these K channels. We conclude that two types of K channel are present in the basolateral membrane of PC.