Pulmonary ischemia-reperfusion results in transient hypertension and edema formation. Implicated in this injury are partially reduced oxygen species including the highly reactive hydroxyl radical. We measured ischemia-reperfusion injury and hydroxyl radical production following 90 min of either air-ventilated, N2-ventilated, or nonventilated ischemia in an isolated rabbit lung preparation. We found that edema formation was independent of alveolar oxygen tension (PO2); all ischemic groups had similar edema formation, regardless of the type of ventilation. Weight gain was 37-50 g of fluid during 40 min of reperfusion. Production of hydroxyl radical, measured by nonenzymatic hydroxylation of salicylate, was influenced by PO2 with a significant increase after air-ventilated ischemia (P < 0.05) but not after N2-ventilated ischemia. Treatment with dimethylthiourea or superoxide dismutase reduced edema formation 60-80% after air (P < 0.05)- and N2 (P < 0.05)-ventilated ischemia, whereas treatment with catalase protected only N2-ventilated ischemia (P < 0.05). Our results implicate two distinct mechanisms by which partially reduced oxygen species may contribute to pulmonary ischemia-reperfusion injury. One is by a mechanism capable of generating hydroxyl radical at normal PO2; the second is from reactions active at low PO2, the products of which are metabolized readily by extracellular enzymatic scavengers. The precise mechanisms of oxidant generation are not clear, but the findings suggest that a complex oxidative injury occurs during ischemia-reperfusion.