K+ channels play an important role in renal collecting duct cell function. The current study examined barium (Ba2+)-sensitive whole-cell K+ currents (IKBa) in mouse isolated collecting duct principal cells. IKBa demonstrated strong inward rectification and was inhibited by Ba2+ in a dose- and voltage-dependent fashion, with the K d decreasing with hyperpolarization. The electrical distance of block by Ba2+ was around 8.5%. As expected for voltage-dependent inhibition, the association constant increased with hyperpolarization, suggesting that the rate of Ba2+ entry was increased at negative potentials. The dissociation constant also increased with hyperpolarization, consistent with the movement of Ba2+ ions into the intracellular compartment at negative potentials. These properties are not consistent with ROMK but are consistent with the properties of Kir2.3. Kir2.3 is thought to be the dominant basolateral K+ channel in principal cells. This study provides functional evidence for the expression of Kir2.3 in mouse cortical collecting ducts and confirms the expression of Kir2.3 in this segment of the renal tubule using reverse-transcriptase polymerase chain reaction. The conductance described here is the first report of a macroscopic K+ conductance in mouse principal cells that shares the biophysical profile of Kir2.3. The properties and dominant nature of the conductance suggest that it plays an important role in K+ handling in the principal cells of the cortical collecting duct.