The aim of the present study was: 1) to determine the effects of the novel selective inhibitor of guanylyl cyclase, ODQ (1H-[1,2,4]oxadiazolo[4,3,-a]quinoxaline-1-one) on basal and agonist-stimulated cyclic GMP levels in cultured porcine aortic endothelial cells and bovine pulmonary artery strips; 2) to determine its effects on agonist-induced relaxations of bovine pulmonary artery strips; and 3) to compare pulmonary artery cyclic GMP levels with vessel relaxation. ODQ (1 nM-30 microM) inhibited cyclic GMP accumulation in endothelial cells stimulated with S-nitrosoglutathione, nitroprusside and 3-morpholine-sydnonimine at IC50 values of 40 to 100 nM. Complete suppression of cyclic GMP generation was observed at approximately 10 microM. Relaxation of pulmonary artery strips induced by S-nitrosoglutathione, nitroprusside, glycerol trinitrate, nitrite (all endothelium-independent), bradykinin and the Ca++ ionophore A23187 (endothelium-dependent) were antagonized by ODQ (1-10 microM) in a concentration-dependent way. A consistent feature of the inhibitor was that maximal relaxant effects also were reduced. Basal levels and agonist-induced increases in arterial tissue cyclic GMP were inhibited in the same concentration range. However, tissue cyclic GMP production correlated poorly with pulmonary artery relaxation in that relaxations induced by S-nitrosoglutathione were only inhibited in part (50%), whereas rises in cyclic GMP were abolished completely by ODQ (10 microM). Furthermore, at 1 microM, ODQ had no effect on relaxation induced by endothelium-dependent agonists, but prevented entirely stimulation of cyclic GMP accumulation in arterial tissue. These results suggest that ODQ inhibits nitrovasodilator-induced and endothelium-dependent relaxation through inhibition of guanylyl cyclase activation, but also point to the presence of a cyclic GMP-independent component of relaxation in bovine pulmonary artery.