Using isolated rabbit lungs (n = 72) perfused at a constant flow of 70 ml/min, we analyzed whether the antioxidant system in erythrocytes significantly contributes to maintaining pulmonary vascular responsiveness to alveolar hypoxia (HPV) in normal lungs. As a measure of HPV, we observed the difference (delta P) between mean pulmonary arterial pressure during ventilation with normoxic gas mixture (21% O2, 5% CO2 in N2) and that during hypoxic gas breathing (3% O2, 5% CO2 in N2). Autologous erythrocytes obtained from the animals treated with various substances inhibiting either superoxide dismutase (SOD), the anion channel of the membrane, catalase (CAT) or glutathione peroxidase (GSH-Px). Subsequently, delta P was systematically measured in the perfusate, whose hematocrit was adjusted to 6-7% with normal or treated erythrocytes as described above. Further, the effects of adding SOD (75 U/ml) or CAT (1000 U/ml) to the perfusate on delta P were examined. The following results were obtained. (1) Inhibition of the superoxide scavenging mechanism in erythrocytes (SOD and anion channel) exerted no significant influence on delta P. (2) Inhibition of hydrogen peroxide scavengers in erythrocytes did not alter the scope of delta P. (3) Addition of either SOD or CAT to the perfusate did not show any significant effect on delta P. The findings are highly consistent with the idea that HPV in normal lungs is essentially independent of antioxidant enzymes in erythrocytes, which are expected to be one of the important factors determining the total capacity to deal with oxidant stress in the lung.