We hypothesized that since added airway pressure compresses bronchial vessels, the airway hyperemia found following airway injury would be reduced by positive end-expiratory pressure (PEEP). Accordingly, we measured the effect of 15 cm H2O PEEP on bronchial and pulmonary blood flows by the radioactive microsphere reference flow technique in closed chested goats (n = 7) before and after aspiration injury to the left lung with 0.1 N HCl. Thirty minutes after aspiration, the pulmonary blood flow to the injured left lung was reduced by one third, whereas the total bronchial blood flow to the left lung (normalized to mean systemic pressure of 100 torr) doubled (11.3 ± 2.2 to 20.6 ± 1.0 ml/mini00 torr; p < 0.01). Increasing PEEP from 5 to 15 cm H2O decreased total bronchial blood flow by about half both before (11.3 ± 2.2 falling to 5.7 ± 1.4 ml/min/100 torr) and after injury (20.6 ± 1.0 falling to 10.3 ± 2.7 ml/min/100 torr). The airway portion (down to 2-3 mm airways) of the total bronchial blood flow of the injured lung increased more than threefold (1.4 ± 0.5 rising to 5.5 ± 1.3 ml/min/100 torr; p < 0.01). This increased flow after aspiration was less affected by PEEP of 15 cm H2O (5.5 ± 1.3 to 2.8 ± 0.7 ml/min/100 torr, p = 0.09) than before injury (1.4 ± 0.5 falling to 0.5 ± 0.1 ml/min/100 torr; p < 0.05). The increase of the parenchymal portion of the bronchial blood flow after injury, although apparent (9.9 ± 1.8 increasing to 15.1 ± 1.2 ml/min/100 torr), was not significant (p = 0.08). However, 15 cm H2O PEEP halved this parenchymal flow (15.1 ± 1.2 to 7.5 ± 2.4ml/min/100 torr; p < 0.05). We conclude that within half an hour after acid aspiration of one lung its pulmonary blood flow falls by one third but its total bronchial blood flow doubles. The bronchial blood flow to the airway, the tissue most injured by the acid, increases more than the bronchial flow to the lung parenchyma. A PEEP level of 15 cm H2O essentially eliminates the compensatory increase in