It has been suggested that hypoxic pulmonary vasoconstriction (HPV) may mainly proceed via loss of normoxic vasodilation, forwarded by tonic O2-dependent formation of nitric oxide and superoxide (23). Both agents may stimulate guanylate cyclase, the latter via conversion to hydrogen peroxide and formation of compound I with catalase. We probed this hypothesis in perfused rabbit lungs, employing the superoxide scavengers superoxide dismutase (SOD), 4,5-dihydroxy-1,3-benzene disulfonic acid (Tiron), and nitro blue tetrazolium (NBT) and the catalase inhibitor aminotriazole (AT). NBT turned out to be a potent dose-dependent inhibitor of HPV in a concentration range of 200 nM to 1 microM, and superimposable dose-inhibition curves were obtained when lung synthesis of nitric oxide and vasodilatory prostanoids was preblocked by NG-monomethyl-L-arginine (L-NMMA) and acetylsalicylic acid (ASA). The NBT effect was specific because no inhibition in the vasoconstrictor responses to the stable thromboxane analog U-46619 and angiotensin II was observed. In contrast, SOD and Tiron were ineffective. AT exerted nonspecific inhibition of the hypoxia- and chemical vasoconstrictor-induced pressor responses. When applied under normoxic conditions, however, NBT alone or coapplied with L-NMMA or ASA, both for blockage of parallel vasodilatory pathways, did not mimic the hypoxia-induced vasoconstrictor response. In conclusion, the present study supports an important role for superoxide in the basic mechanism of HPV, but it questions the concept that loss of tonic vasorelaxation via this pathway is the underlying event in rabbit lungs. The mechanisms relating O2 tension-dependent superoxide and hydrogen peroxide generation to the vasoconstrictor event occurring in HPV remain to be further elucidated.