During cardiopulmonary bypass (CPB), neutrophils (PMNs) may be stimulated by shear stress which could contribute to the pulmonary injury that occurs after CPB. To elucidate whether mechanically stimulated PMNs increase pulmonary vascular permeability, measured as the pulmonary filtration coefficient (K) and pulmonary vascular resistance, and to elucidate whether superoxide anion mediates this increase, we assessed the effects of stimulated and unstimulated PMNs, and of superoxide dismutase (SOD) on K and resistance in isolated perfused lungs from Sprague-Dawley rats. PMNs were stimulated by gentle agitation in a glass vial for 10 s. Lungs perfused with the stimulated PMNs, being the stimulated group (n = 6), elicited a 5-fold increase in the filtration coefficient compared with lungs perfused with unstimulated cells, being the unstimulated group (n = 6). This increase in filtration was completely blocked by the preincubation of stimulated PMNs with CD18 monoclonal antibody, being the Ab group (n = 6), and also by superoxide dismutase, being the SOD group (n = 6). Pulmonary vascular resistance was not increased by stimulated PMNs, and the accumulation of stimulated PMNs was not blocked by SOD. These findings suggest that stimulated PMNs increase K and that superoxide anion may injure the pulmonary vascular endothelial cells.