To study the role of cyclooxygenase metabolites in changes in the pulmonary vasculature induced by mechanically activated white blood cells (WBCs), the effects of activated and inactive WBCs, and of a cyclooxygenase inhibitor, were studied in isolated perfused lungs from Sprague-Dawley rats. WBCs were activated by gentle agitation in a glass container for 10s. Baseline measurements were made, and then activated or inactive WBCs were added to the perfusate. Perfusion was stopped for 90 minutes, and then started again. The effects of the cyclooxygenase inhibitor meclofenamate on the pulmonary vascular filtration coefficient and on pulmonary vascular resistance were also measured. In the group that received activated WBCs, the pulmonary vascular filtration coefficient and the pulmonary vascular resistance were about 2.5 times and 3.3 times higher, respectively, than those in the group that received inactive WBCs. However, this apparent increase in the filtration coefficient caused by activated WBCs was partly blocked by meclofenamate. Histological examination indicated that meclofenamate did not prevent the adhesion of WBCs to the pulmonary vascular endothelium. These date indicate that WBCs that have been made to adhere to vessel walls can induce pulmonary vascular injury via cyclooxygenase products.