—Sepsis and endotoxemia impair hypoxic pulmonary vasoconstriction (HPV), thereby reducing systemic oxygenation. To assess the role of leukotrienes (LTs) in the attenuation of HPV during endotoxemia, the increase in left lung pulmonary vascular resistance (LPVR) before and during left mainstem bronchus occlusion (LMBO) was measured in mice with and without a deletion of the gene encoding 5-lipoxygenase (5-LO). LMBO increased the LPVR equally in saline-challenged wild-type and 5-LO–deficient mice (96±20% and 94±19%, respectively). Twenty-two hours after challenge with Escherichia coli endotoxin, the ability of LMBO to increase LPVR was markedly impaired in wild-type mice (27±7%; P<0.05) but not in 5-LO–deficient mice (72±9%) or in wild-type mice pretreated with MK886, an inhibitor of 5-LO activity (76±10%). Compared with wild-type mice, endotoxin-induced disruption of lung structures and inflammatory cell influx in the lung were markedly attenuated in 5-LO–deficient mice. Administration of MK571, a selective cysteinyl LT₁ receptor antagonist, 1 hour before endotoxin challenge preserved HPV and attenuated pulmonary injury in wild-type mice but did not prevent the endotoxin-induced increase in pulmonary myeloperoxidase activity. Taken together, these findings demonstrate that a 5-LO product, most likely a cysteinyl LT, contributes to the attenuation of HPV and to pulmonary injury after challenge with endotoxin.