Endothelium-dependent hyperpolarizations and relaxation of vascular smooth muscle induced by acetylcholine and bradykinin are mediated by endothelium-derived hyperpolarizing factors (EDHFs). In bovine coronary arteries, arachidonic acid metabolites, epoxyeicosatrienoic acids (EETs), function as EDHFs. The 14,15-EET analog 14,15-epoxyeicosa-5(Z)-enoic-methylsulfonylimide (14,15-EEZE-mSI) was synthesized and tested for agonist and antagonist activity. In U46619-preconstricted bovine coronary arterial rings, 14,15-, 11,12-, 8,9-, and 5,6-EET induced maximal concentration-related relaxation averaging 75% to 87% at 10 μmol/L, whereas, 14,15-EEZE-mSI induced maximal relaxation averaging only 7%. 14,15-EEZE-mSI (10 μmol/L) preincubation inhibited relaxation to 14,15- and 5,6- EET but not 11,12- or 8,9- EET. 14,15-EEZE-mSI also inhibited indomethacin-resistant relaxation to arachidonic acid and indomethacin-resistant and l-nitroarginine-resistant relaxation to bradykinin and methacholine. It did not alter the relaxation to sodium nitroprusside, iloprost, or the K⁺ channel openers bimakalim or NS1619. In cell-attached patches of isolated bovine coronary arterial smooth muscle cells, 14,15-EEZE-mSI (100 nmol/L) blocked the 14,15-EET–induced (100 nmol/L) activation of large-conductance, calcium-activated K⁺ channels. Mass spectrometric analysis of rat renal cortical microsomes incubated with arachidonic acid showed that 14,15-EEZE-mSI (10 μmol/L) increased EET concentrations while decreasing the concentrations of the corresponding dihydroxyeicosatrienoic acids. Therefore, 14,15-EEZE-mSI inhibits relaxation to 5,6- and 14,15- EET and the K⁺ channel activation by 14,15-EET. It also inhibits the EDHF component of bradykinin-induced, methacholine-induced, and arachidonic acid–induced relaxation. These results suggest that 14,15- or 5,6 -EET act as an EDHF in bovine coronary arteries.