Endothelium-dependent relaxation cannot be fully attributed to the release of nitric oxide or prostacyclin (PGI2). In resistance-sized vessels and coronary arteries a high proportion of endothelium-dependent relaxation, in response to agonist-induced or mechanical stimulation of endothelial cells, can be attributed to the release of 1 or more endothelium-derived hyperpolarizing factor (EDHF). In coronary arteries EDHF has been pharmacologically characterized as a cytochrome P450 (CYP)-derived metabolite of arachidonic acid. We show here that a CYP 2C arachidonic acid epoxygenase, homologous to CYP 2C8/9, is expressed in cultured human endothelial cells and native porcine coronary artery endothelial cells. Down-regulation of CYP 2C protein by transfection of porcine coronary arteries with anti-sense oligonucleotides decreased EDHF-mediated vascular responses while EDHF-mediated hyperpolarisation and relaxation were potentiated by the CYP-inducing compound beta-naphthoflavone. Thus, CYP 2C appears to play a crucial role in the generation of EDHF-mediated responses in porcine coronary arteries.