We have cloned a novel inward rectifier potassium channel from a rat brain cDNA library and designated it RB-IRK2. The rat brain cDNA library was screened using a fragment of the mouse macrophage IRK1 cDNA as a probe. The amino acid sequence of RB-IRK2 shares 70%, 40% and 45% identity to mouse IRK1, rat ROMK1 and rat GIRK1, respectively. Xenopus oocytes injected with cRNA derived from RB-IRK2 expressed a potassium current which showed inward-rectifying channel characteristics similar to the IRK1 current, but distinct from the ROMK1 or the GIRK1 currents. However, the localization of RB-IRK2 mRNA in rat tissues, assessed by the Northern blot analysis, differed from that of mouse IRK1. These results indicate that the IRK family is composed of multiple genes, which express in different tissues and therefore may play heterogenous functional roles in various organs, including rat central nervous system.