We investigated the effects of lung injury due to alpha-naphthylthiourea (ANTU) on pulmonary vascular reactivity. Rats were treated with ANTU (10 mg/kg ip) or the vehicle Tween 80. Four hours later, lungs from ANTU-treated rats had increased wet-to-dry weight ratios, bronchial lavage protein concentrations, and perivascular edema. To test vascular reactivity, lungs were isolated and perfused with blood at constant flow rate, while mean pulmonary arterial pressure was monitored. ANTU-treated lungs vasoconstricted earlier than Tween-treated lungs in response to severe airway hypoxia (fractional inspired O2 0%). ANTU-treated lungs vasoconstricted in response to 10% O2, while Tween-treated lungs failed to respond to 10% O2, indicating that the threshold for hypoxic vasoconstriction was decreased by ANTU. ANTU also decreased the threshold for and increased the magnitude of angiotensin II pressor responses, indicating that the increased vasoreactivity was not specific for hypoxia. Addition of meclofenamate to perfusates increased the rate and magnitude of responses to 0% O2 in Tween-treated lungs, but did not change the responses of ANTU-treated lungs. Light microscopy of ANTU-treated lungs showed no pulmonary arterial obstruction, and electron microscopy revealed mild capillary endothelial cell injury. We conclude that enhanced pulmonary vascular reactivity accompanies the increased-permeability pulmonary edema caused by ANTU. A similar increase in vasoreactivity might contribute to pulmonary hypertension observed in patients with the adult respiratory distress syndrome.