We investigated the effects of hypoxia/reoxygenation exposure on the barrier function of endothelial cell monolayers. Bovine pulmonary microvessel endothelial cells were grown to confluence on microporous filters (0.8-μm pore diameter) and exposed to hypoxia (0.1% O2or Po2∼1 mm Hg) for 2, 4, 12, or 24 hours, followed by reoxygenation with room air for a period ranging from 16 seconds to 2 hours. The transendothelial clearance rate of125I-albumin was measured to determine the permeability of endothelial monolayers. Permeability increased twofold or fivefold over control values after 1 hour of reoxygenation in monolayers that had been exposed to either 12 or 24 hours of hypoxia. The response occurred within 5 minutes of reoxygenation, increased maximally by 40 minutes, and remained elevated with continuous reoxygenation for up to 2 hours. The increase in permeability was associated with F-actin reorganization, a change to spindlelike cells, and injured mitochondria. Immunoblot analysis indicated that neither hypoxia alone nor reoxygenation changed CuZn superoxide dismutase (SOD), MnSOD, and catalase levels. However, release of superoxide anions (O2−) into the extracellular medium increased by twofold within 40–60 minutes of reoxygenation. Treatment of endothelial cells with CuZnSOD (100 units/ml) for the 24-hour hypoxia period prevented O2−generation and ∼50% of the increase in permeability. Higher CuZnSOD concentrations (≥200 units/ml) were not protective. Treatment with catalase (100–1,000 units/ml) inhibited the reoxygenation-induced increase in permeability at the highest catalase concentration (1,000 units/ml), suggesting a critical role of hydrogen peroxide in mediating the response. We conclude that reoxygenation of endothelial cells causes the generation of oxidants and that this mediates the increase in vascular endothelial permeability occurring during reoxygenation. The loss of endothelial barrier function may contribute to the pathogenesis of reperfusion tissue injury. (Circulation Research1992;70:991–998)