1. The mechanisms underlying the vasodilator response to urocortin are incompletely understood. The present study was designed to examine the role of endothelial nitric oxide and Ba(2+)-sensitive K(+) channels in the endothelium-dependent component of urocortin-induced relaxation in the rat left anterior descending coronary artery. 2. Urocortin induced both endothelium-dependent and -independent relaxation with respective pD(2) of 8.64+/-0.03 and 7.90+/-0.10. Removal of endothelium reduced the relaxing potency of urocortin. In rings pretreated with 10(-4) M N(G)-nitro-L-arginine methyl ester, 10(-5) M methylene blue or 10(-5) M ODQ, the urocortin-induced relaxation was similar to that observed in endothelium-denuded rings. L-Arginine (5x10(-4) M) antagonized the effect of N(G)-nitro-L-arginine methyl ester. 3. The relaxant response to urocortin was reduced in endothelium-intact rings preconstricted by 3.5x10(-2) M K(+) and abolished when extracellular K(+) was raised to 5x10(-2) M. Pretreatment with 10(-4) M BaCl(2) significantly inhibited urocortin-induced relaxation. Combined treatment with 10(-4) M BaCl(2) plus 10(-4) M N(G)-nitro-L-arginine methyl ester did not cause further inhibition. In urocortin (10(-8) M)-relaxed rings, BaCl(2) induced concentration-dependent reversal in vessel tone. Tertiapin-Q (10(-6) M) also attenuated urocortin-induced relaxation. In contrast, BaCl(2) did not alter urocortin-induced relaxation in endothelium-denuded rings. 4. In endothelium-denuded rings, hydroxylamine- and nitroprusside-induced relaxation was inhibited by 10(-4) M BaCl(2), but not by 10(-6) M tertiapin-Q. 5. The endothelium of the coronary artery was moderately stained with the antiserum against urocortin. 6. Taken together, the present results indicate that the urocortin-induced endothelium-dependent relaxation of rat coronary arteries is likely attributable to endothelial nitric oxide and subsequent activation of Ba(2+)- or tertiapin-Q-sensitive K(+) channels. The urocortin-induced endothelium-dependent relaxation appears to be mediated by cyclic GMP-dependent mechanisms.