1. Previous studies, demonstrated that endothelium-dependent relaxations which are resistant to nitro-L-arginine (an inhibitor of nitric oxide synthase) are accompanied by membrane hyperpolarization in the porcine coronary artery. The present experiments were designed to characterize further this type of endothelium-dependent relaxation in response to bradykinin by measuring isometric force in isolated rings of that artery. The experiments were performed in the presence of indomethacin to rule out vasoactive prostanoids. 2. Bradykinin induced comparable endothelium-dependent relaxations of proximal and distal rings of porcine coronary arteries contracted with prostaglandin F2 alpha in the presence of nitro-L-arginine. 3. Bradykinin and SIN 1 (a donor of nitric oxide) reduced contractions induced by prostaglandin F2 alpha in an additive fashion in the presence of nitro-L-arginine. 4. Bradykinin (in the presence of nitro-L-arginine) relaxed the tissues contracted with tetraethylammonium, prostaglandin F2 alpha, phorbol 12, 13-diacetate or endothelin, with similar pD2 values. 5. The time course of the relaxations induced by bradykinin (in the presence of nitro-L-arginine) and UK14304 (an alpha 2-adrenoceptor agonist, in the absence of the inhibitor of nitric oxide synthase) were comparable. 6. These results suggest that, in the porcine coronary artery, nitro-L-arginine-resistant relaxations (a) are distributed similarly in the proximal and distal parts of the artery, (b) contribute to inhibition of vascular smooth muscle with nitric oxide in an additive fashion, (c) occur during contractions induced by various contractile agents and (d) do not precede those mediated by nitric oxide.