Aim: Endothelium-dependent relaxations to certain neurohumoral substances are mediated by pertussis toxin-sensitive Gi/o protein. Our experiments were designed to determine the role, if any, of pertussis toxin-sensitive G-proteins in relaxations attributed to endothelium-derived hyperpolarizing factor (EDHF). Methods: Pig coronary arterial rings with endothelia were suspended in organ chambers filled with Krebs-Ringer bicarbonate solution maintained at 37℃ and continuously aerated with 95%O2 and 5% CO2. Isometric tension was measured during contractions to prostaglandin F2, in the presence of indomethacin and Nω-nitro-L-arginine methyl ester (L-NAME). Results: Thrombin, the thrombin re-ceptor-activating peptide SFLLRN, bradykinin, substance P, and calcimycin pro-duced dose-dependent relaxations. These relaxations were not inhibited by prior incubation with pertussis toxin, but were abolished upon the addition of charyb-dotoxin plus apamin. Relaxations to the α2-adrenergic agonist UK14304 and those to serotonin were abolished in the presence of indomethacin and L-NAME. Conclusion: Unlike nitric oxide-mediated relaxations, EDHF-mediated relax-ations of pig coronary arteries do not involve pertussis toxin-sensitive pathways and are Gi/o protein independent.