The influence of stimulated polymorphonuclear neutrophils on pulmonary arterial pressure and vascular leakage in isolated perfused rat lungs was investigated. We exposed isolated neutrophils to various stimuli in vitro, instilled the cells in the lung perfusate, and studied the effects on pulmonary arterial pressure and passage of fluorescently labeled dextran (4100 dalton) from the pulmonary circulation into the lung. We found that neutrophils stimulated with the calcium ionophore A23187 or with E. coli endotoxin had no significant influence on the pressure or the passage of dextran. On the other hand, neutrophils preincubated with endotoxin and then stimulated with A23187 caused significant increases, both in pulmonary arterial pressure and accumulation of dextran in the lung. Both these effects were attenuated by BN 52021, a specific platelet-activating factor antagonist, and by nordihydroguaiaretic acid, an agent that inhibited the generation of platelet-activating factor in A23187-stimulated neutrophils. These findings demonstrate that activated neutrophils can increase pulmonary arterial pressure and lung fluid accumulation and suggest that endotoxin-stimulated activated neutrophils exert at least some of their action via generation of platelet-activating factor.