P-450 metabolites, including the epoxyeicosatrienoic acids, are likely candidates for endothelial derived hyperpolarising factor (EDHF). In the present study, we confirm that the stable analogue 11-nonyloxyundec-8(Z)-enoic acid is a vasodilator of murine vessels. However, we also show that the ‘epoxyeicosatrienoic acid receptor’ antagonist 14,15 EEZE similarly dilates murine vessels contracted with U46619, prostaglandin F2α or methoxamine, but not with endothelin-1 or potassium. We suggest that 14,15 EEZE is a partial agonist for the epoxyeicosatrienoic acids/EDHF receptor. These results illustrate an important pharmacological property of this antagonists, which is being increasingly used to study the nature of EDHF.