Kir5.1 is an inwardly rectifying K+ channel (Kir) subunit, whose physiological function is unknown. Human embryonic kidney HEK293T cells co-transfected with rat Kir5.1 and Kir4.1 cDNA expressed a functional K+ channel, whose properties were significantly different from those of the homomeric Kir4.1 channel. Formation of a Kir4. 1/Kir5.1 assembly in HEK293T was confirmed biochemically. We found that heteromeric Kir4.1/Kir5.1 channel activity was affected by internal pH levels between 6.0 and 8.0, when the homomeric Kir4.1 channel activity was relatively stable. Changing external pH in this range had no effect on either Kir channel. Western blot analysis using specific antibodies revealed that Kir4.1 and Kir5.1 proteins were expressed in kidney and brain, but co-immunoprecipitated only from kidney. These results indicate that the co-assembly of Kir5.1 with Kir4.1 occurs in vivo, at least in kidney. The heteromeric Kir4. 1/Kir5.1 channel may therefore sense intracellular pH in renal epithelium and be involved in the regulation of acid-base homeostasis.