11,12-epoxyeicosatrienoic acid (11,12-EET), a potent vasodilator produced by the endothelium, acts on calcium-activated potassium channels, and shares biological activities with the heme oxygenase/carbon monoxide (HO/CO) system. We examined whether activation of HO mediates the dilator action of 11,12-EET, and that of the other EETs, on rat mesenteric arteries. Dose-response curves (10^-9-10^-6 M) to 5,6-EET, 8,9-EET, 11,12-EET, 14,15-EET, and acetylcholine (ACh) (10^-9 to 10^-4 M) were evaluated in preconstricted (10^-6 mol/l phenylephrine) mesenteric arteries (<350 µm diameter) in the presence or absence of 1) cyclooxygenase inhibitor, indomethacin (2.8 µM), 2) the HO inhibitor, chromium mesoporphyrin (CrMP) (15 µM), 3) the soluble guanylyl cyclase (GC) inhibitor, ODQ (10 µM), and 4) the calcium activated potassium channel (K_Ca) inhibitor, iberiotoxin (25 nM). The vasodilator response to 11,12-EET was abolished by CrMP and iberiotoxin, while indomethacin and ODQ had no effect. In contrast, the effect of ACh was attenuated by ODQ, but not by CrMP. The vasodilator effect of 8,9-EET like that of 11,12-EET was greatly attenuated by HO inhibition. In contrast, the mesenteric vasodilator response to 5,6-EET was independent of both HO and GC, while that to 14,15-EET demonstrated two components, an HO and a GC, of equal magnitude. Incubation of mesenteric microvessels with 11,12-EET caused a 30% increase in CO release, an effect abolished by inhibition of HO. We conclude that the rat mesenteric vasodilator action of 11,12-EET is mediated via an increase in HO activity and an activation of K_Ca channels.