Pneumonia was induced in rats by instillation of carrageenin (0.5 ml of 0.7% solution) into the trachea. Three or four days after instillation, the lungs were isolated, perfused with blood of healthy rat blood donors, and ventilated with air + 5% CO2 or with various hypoxic gas mixtures. Pulmonary vascular reactivity to acute hypoxic challenges was significantly lower in lungs of rats with pneumonia than in lungs of controls. The relationship between O2 concentration in the inspired gas and Po2 in the blood effluent from the preparation was shifted significantly to lower Po2 in lungs with pneumonia compared to control ones. These changes were not present in rats allowed to recover for 2-3 weeks after carrageenin instillation. We suppose that blunted hypoxic pulmonary vasoconstriction may contribute to hypoxaemia during acute pulmonary inflammation. Decreased Po2 in the blood effluent from the isolated lungs with pneumonia implies significant increase of oxygen consumption by the cells involved in the inflammatory process.