1 The possibility that the endothelium-derived hyperpolarising factor (EDHF) in the rat hepatic artery is a cytochrome P450 mono-oxygenase metabolite of arachidonic acid was examined in the present study. In this preparation, acetylcholine elicits EDHF-mediated relaxations in the presence of the nitric oxide (NO) synthase and cyclo-oxygenase inhibitors No-nitro-L-arginine (L-NOARG) and indomethacin, respectively.2 17-Octadecynoic acid (17-ODYA, 50 gM), a suicide-substrate inhibitor of the cytochrome P450 mono-oxygenases responsible for the production of 5, 8, 11,[12][13][14], had no effect on acetylcholine-induced relaxations in the presence of L-NOARG (0.3 mM) plus indomethacin (10 gM). Furthermore, 5, 8, 11, Clotrimazole (3 gM) was without effect on these responses. The relaxant actions of the NO donor, 3-morpholino-sydnonimine, were unaffected by proadifen (10 gM). 5 The relaxant effects of the opener of ATP-sensitive potassium channels, levcromakalim, were abolished by proadifen (10 gM) and strongly attenuated by clotrimazole (3 gM). Proadifen (10 gM) also abolished the hyperpolarization induced by levcromakalim (1 gM). 6 The lack of effect of 17-ODYA on relaxations mediated by EDHF, together with the failure of extracellularly-applied EETs to produce relaxation, collectively suggest that EDHF is not an EET in the rat hepatic artery. It seems likely that inhibition of ion channels in the smooth muscle rather than reduced EDHF formation in the endothelium offers a better explanation for the actions of the cytochrome P450 inhibitors proadifen and clotrimazole.