Endothelium-dependent hyperpolarization and relaxation of vascular smooth muscle are mediated by endothelium-derived hyperpolarizing factors (EDHFs). EDHF candidates include cytochrome P-450 metabolites of arachidonic acid, K⁺, hydrogen peroxide, or electrical coupling through gap junctions. In bovine coronary arteries, epoxyeicosatrienoic acids (EETs) appear to function as EDHFs. A 14,15-EET analogue, 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EEZE) was synthesized and identified as an EET-specific antagonist. In bovine coronary arterial rings preconstricted with U46619, 14,15-EET, 11,12-EET, 8,9-EET, and 5,6-EET induced concentration-related relaxations. Preincubation of the arterial rings with 14,15-EEZE (10 μmol/L) inhibited the relaxations to 14,15-EET, 11,12-EET, 8,9-EET, and 5,6-EET but was most effective in inhibiting 14,15-EET–induced relaxations. 14,15-EEZE also inhibited indomethacin-resistant relaxations to methacholine and arachidonic acid and indomethacin-resistant and l-nitroarginine-resistant relaxations to bradykinin. It did not alter relaxation responses to sodium nitroprusside, iloprost, or the K⁺ channel activators (NS1619 and bimakalim). Additionally, in small bovine coronary arteries pretreated with indomethacin and l-nitroarginine and preconstricted with U46619, 14,15-EEZE (3 μmol/L) inhibited bradykinin (10 nmol/L)–induced smooth muscle hyperpolarizations and relaxations. In rat renal microsomes, 14,15-EEZE (10 μmol/L) did not decrease EET synthesis and did not alter 20-hydroxyeicosatetraenoic acid synthesis. This analogue acts as an EET antagonist by inhibiting the following: (1) EET-induced relaxations, (2) the EDHF component of methacholine-induced, bradykinin-induced, and arachidonic acid–induced relaxations, and (3) the smooth muscle hyperpolarization response to bradykinin. Thus, a distinct molecular structure is required for EET activity, and alteration of this structure modifies agonist and antagonist activity. These findings support a role of EETs as EDHFs.