Endothelial cells synthesize and release vasoactive mediators in response to various neurohumoural substances (e.g. bradykinin or acetylcholine) and physical stimuli (e.g. cyclic stretch or fluid shear stress). The best-characterized endothelium-derived relaxing factors are nitric oxide and prostacyclin. However, an additional relaxant pathway associated with smooth muscle hyperpolarization also exists. This hyperpolarization was originally attributed to the release of an endothelium-derived hyperpolarizing factor (EDHF) that diffuses to and activates smooth muscle K⁺ channels. More recent evidence suggests that endothelial cell receptor activation by these neurohumoural substances opens endothelial cell K⁺ channels. Several mechanisms have been proposed to link this pivotal step to the subsequent smooth muscle hyperpolarization. The main concepts are considered in detail in this review.