Hydrogen peroxide can be released by different cells such as the nerves, the endothelial or phagocytotic white blood cells which can all interact with vascular smooth muscles. We show that hydrogen peroxide hyperpolarizes and relaxes pig coronary artery smooth muscle cells. The possibility that the endothelium derived hyperpolarizing factor released by the endothelium in response to bradykinin and substance P being hydrogen peroxide was tested using catalase, an enzyme which hydrolyses hydrogen peroxide. We find that this particular endothelial hyperpolarizing factor and hydrogen peroxide are two distinct molecules.