Renal collecting duct K+ channels play a pivotal role in K+ handling. Work has identified strong, inwardly rectifying K+ currents in mouse cortical collecting duct (CCD). Previous studies have shown that mRNA for Kir 2.1, 2.2 and 2.3 are found in mouse and rat kidney. The current study determined whether mRNA for these channels was present in mouse CCDs. C57/B6 mice were sacrificed by cervical dislocation, according to UK legislation. Cortical tubules were isolated by enzyme digestion. RT-PCR analysis was carried out on unsorted tubules (whole cortex) or on CCDs (identified by the presence of bifurcations). The expression of mRNA for ENaC (collecting duct), uromodulin (thick ascending limb), [gamma]GT (proximal tubule) and Kir2.1, 2.2 and 2.3 was examined. All experiments were repeated on at least three separate samples. Reactions were performed in the presence or absence of reverse transcriptase enzyme and products digested with a restriction endonuclease to confirm identity. In whole cortex mRNA for ENaC, uromodulin, [gamma]GT, Kir2.1, 2.2 and 2.3 was observed. In contrast, only mRNA for ENaC and Kir2.3 was observed in CCD samples. The expression of ENaC is consistent with a CCD identity. The expression of mRNA for Kir2.1, 2.2 and 2.3 in whole cortex supports previous studies. The current study shows, however, that only mRNA for Kir2.3 is found in CCDs, suggesting that Kir2.3 is a strong candidate for the inwardly rectifying K+ currents previously identified.