We used the isolated-perfused rat lung model to study the influence of pulmonary ventilation and surfactant instillation on the development of postreperfusion lung microvascular injury. We hypothesized that the state of lung inflation during ischemia contributes to the development of the injury during reperfusion. Pulmonary microvascular injury was assessed by continuously monitoring the wet lung weight and measuring the vessel wall ¹²⁵I-labeled albumin (¹²⁵I-albumin) permeability-surface area product (PS). Sprague-Dawley rats (n = 24) were divided into one control group and five experimental groups (n = 4 rats per group). Control lungs were continuously ventilated with 20% O₂ and perfused for 120 min. All lung preparations were ventilated with 20% O₂ before the ischemia period and during the reperfusion period. The various groups differed only in the ventilatory gas mixtures used during the flow cessation: group I, ventilated with 20% O₂ ; group II, ventilated with 100% N₂; group III, lungs remained collapsed and unventilated; group IV, same as group III but pretreated with surfactant (4 ml/kg) instilled into the airway; and group V, same as group III but saline (4 ml/kg) was instilled into the airway. Control lungs remained isogravimetric with baseline ¹²⁵I-albumin PS value of 4.9 ± 0.3 × 10³ ml · min¹ · g wet lung wt¹. Lung wet weight in group III increased by 1.45 ± 0.35 g and albumin PS increased to 17.7 ± 2.3 × 10³, indicating development of vascular injury during the reperfusion period. Lung wet weight and albumin PS did not increase in groups I and II, indicating that ventilation by either 20% O₂ or 100% N₂ prevented vascular injury. Pretreatment of collapsed lungs with surfactant before cessation of flow also prevented the vascular injury, whereas pretreatment with saline vehicle had no effect. These results indicate that the state of lung inflation during ischemia (irrespective of gas mixture used) and supplementation of surfactant prevent reperfusion-induced lung microvascular injury.