Endothelial cells express a diverse array of ion channels including members of the strong inward rectifier family composed of K 2 subunits. These two-membrane spanning domain channels are modulated by their lipid environment, and exist in macromolecular signaling complexes with receptors, protein kinases and other ion channels. Inward rectifier K channel (K ) currents display a region of negative slope conductance at membrane potentials positive to the K equilibrium potential that allows outward current through the channels to be activated by membrane hyperpolarization, permitting K to amplify hyperpolarization induced by other K channels and ion transporters. Increases in extracellular K concentration activate K allowing them to sense extracellular K concentration and transduce this change into membrane hyperpolarization. These properties position K to participate in the mechanism of action of hyperpolarizing vasodilators and contribute to cell-cell conduction of hyperpolarization along the wall of microvessels. The expression of K in capillaries in electrically active tissues may allow K to sense extracellular K , contributing to functional hyperemia. Understanding the regulation of expression and function of microvascular endothelial K will improve our understanding of the control of blood flow in the microcirculation in health and disease and may provide new targets for the development of therapeutics in the future.