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 N omega-nitro-L-arginine (L-NOARG) and indomethacin, respectively. 2. 17-Octadecynoic acid (17-ODYA, 50 microM), a suicide-substrate inhibitor of the cytochrome P450 mono-oxygenases responsible for the production of 5,6-, 8,9-, 11,12- and 14,15-epoxyeicosatrienoic acids (EETs), had no effect on acetylcholine-induced relaxations in the presence of L-NOARG (0.3 mM) plus indomethacin (10 microM). Furthermore, 5,6-, 8,9-, 11,12- and 14,15- EETs failed to relax arteries without endothelium in the presence of L-NOARG plus indomethacin. 3. Proadifen and clotrimazole, which are inhibitors of several isoforms of cytochrome P450 mono-oxygenases, inhibited acetylcholine-induced relaxations in the presence of L-NOARG plus indomethacin. The concentration of acetylcholine which caused half-maximal relaxation was about 3 and 30 times higher in the presence than in the absence of clotrimazole (3 microM) and proadifen (10 microM), respectively. The maximal relaxation was reduced by proadifen but not by clotrimazole. Proadifen (10 microM) also inhibited acetylcholine-induced hyperpolarization in the presence of L-NOARG plus indomethacin. 4. In the presence of 30 mM K+ plus indomethacin (10 microM), acetylcholine induced an L-NOARG-sensitive relaxation mediated via release of NO. Under these conditions, proadifen (10 microM) shifted the acetylcholine concentration-response curve 6 fold to the right without affecting the maximal relaxation. Clotrimazole (3 microM) was without effect on these responses. The relaxant actions of the NO donor, 3-morpholino-sydnonimine, were unaffected by proadifen (10 microM). 5. The relaxant effects of the opener of ATP-sensitive potassium channels, levcromakalim, were abolished by proadifen (10 microM) and strongly attenuated by clotrimazole (3 microM). Proadifen (10 microM) also abolished the hyperpolarization induced by levcromakalim (1 microM). 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.