Endothelium-derived hyperpolarizing factor (EDHF) plays an important role in modulating vascular tone. Although several candidates for EDHF have been proposed, we have demonstrated that endothelium-derived hydrogen peroxide(H2(0)2) is an EDHF in mouse and human mesenteric arteries and porcine coronary microvessels, which was subsequently confirmed by other investigators in human and canine coronary microvessels. We have also demonstrated that endothelial nitric oxide synthase (eNOS) is a major source of EDHF/H2(0)2, where Cu, Zn-SOD is involved. Furthermore, we showed that genetic disruption of all three NOS isoforms abolishes EDHF responses in mice and that endothelial NOSs system has diverse vasodilator functions depending on the vessel size, mainly contributing to EDHF/H2(0)2 responses in microvessels while serving as a NO-generating system in large arteries.