A 5.1 kb cDNA encoding an inward rectifier K+ channel (BIK) was isolated from a bovine aortic endothelial cell library. The cDNA codes for a 427-amino-acid protein with two putative transmembrane regions. Sequence analysis reveals that BIK is a member of the Kir2.1 family of inward rectifier K+ channels. Expression in Xenopus oocytes showed that BIK is a K+-specific strong inward rectifier channel that is sensitive to extracellular Ba2+, Cs+, and a variety of anti-arrhythmic agents. Northern analysis revealed that endothelial cells express a 5.5 kb BIK mRNA that is sensitive to shear stress.