Endothelial cells produce C-type natriuretic peptide (CNP), which has been proposed as an endothelium-derived hyperpolarizing factor. In porcine coronary arteries, we investigated the vasodilatory effects of CNP and compared them with endothelium-dependent relaxations and hyperpolarizations to bradykinin. Isolated epicardial porcine coronary arteries were studied in organ chambers, and concentration-response curves to CNP and bradykinin were obtained. Membrane potential was measured in endothelial cells and smooth muscle of intact procine coronary arteries during stimulation with CNP or bradykinin. In precontracted porcine coronary arteries with CNP or bradykinin. In precontracted porcine coronay arteries with or without endothelium, CNP (10^-10-10^-6M) evoked relaxations (maximum 42 ± 4%) smaller than those evoked by bradykinin (100 ± 1%), blunted in preparations contracted by KCl instead of U46619 (9,11-dideoxy-11 a,9a-epoxymethano-prostaglandin F_2α; p < 0.05) and unaffected by inhibition of NO synthase (NS). CNP evoked hyperpolarization of vascular smooth muscle of similar magnitude in endothelium-intact (-4.4 ± 1 mV) and endothelium-denuded (-4.6 ± 1 mV) porcine coronary arteries. Bradykinin (10^-10- 10^-6 M) evoked concentration-dependent relaxations in preparations with endothelium only. Although atrial natriuretic peptide-receptor antagonist HS-142-1 (25 μM) slightly reduced the sensitivity to bradykinin (log shift at IC₅₀, twofold; p <0.05), it had no effect on the maximal response to bradykinin. Inhibition of NO synthase partially attenuated, whereas high potassium chloride (30 mM) markedly inhibited relaxations to bradykinin (p < 0.05). Hyperpolarization to bradykinin was much more pronounced than that to CNP (-17 ± 3 mV; p < 0.05 vs. CNP) and was observed in endothelium-intact preparations only and unaffected by HS-142-1. In conclusion, in contrast to bradykinin, CNP induces endothelium-independent and weaker relaxation and hyperpolarization of coronary artery vascular smooth muscle, suggesting that CNP is an unlikely mediator of endothelium-dependent hyperpolarization of porcine coronary arteries.