Although nitric oxide appears to be the major endothelium-derived relaxing factor (EDRF), it cannot explain all endothelium-dependent responses of isolated arteries. Thus, acetylcholine causes an endothelium-dependent, transient hyperpolarization, which is due to the release from the endothelial cells of a diffusible substance (endothelium-derived hyperpolarizing factor, EDHF) other than nitric oxide. The muscarinic receptors on the endothelium that trigger the release of EDHF belong to the M<sub>1</sub>-muscarinic subtype, while those activating the liberation of EDRF are M<sub>2</sub>-muscarinic in nature. The importance of endothelium-dependent hyperpolarization varies among different blood vessels. The hyperpolarization, and the resulting relaxation caused by EDHF can be attributed to an increase in K<sup>+</sup> conductance in the vascular smooth muscle. Although the nature of EDHF remains elusive, it may be a labile metabolic of arachidonic acid.