Leukotriene synthesis inhibition and receptor blockade do not inhibit hypoxic pulmonary vasoconstriction in sheep. Several lines of evidence suggest that leukotrienes may be mediators of hypoxic pulmonary vasoconstriction (HPV). However, the effect of leukotriene inhibition on HPV remains controversial. The present study investigated the effect of leukotriene synthesis inhibition and receptor blockade on HPV in the halothane-anesthetized sheep. After initial baseline measurements, the pulmonary pressor response to 15 min of global hypoxia (FIO2 = 0.13) was measured. A second set of baseline measurements was obtained and the sheep then received the combined cyclooxygenase/lipoxygenase inhibitor BW755C, the selective lipoxygenase inhibitor U60257, or the leukotriene receptor antagonist LY171883. Hemodynamic measurements were obtained after drug administration and during a subsequent hypoxic challenge (FIO2 = 0.13). Initial hypoxic challenge increased pulmonary artery pressure 68% and increased pulmonary vascular resistance 104%. Pulmonary hemodynamics after recovery from hypoxia were similar to initial baseline values. Drug administration had no significant hemodynamic effect. Hypoxic challenge after drug administration resulted in a pulmonary pressor response identical to the initial hypoxic challenge. Because leukotriene synthesis inhibition and receptor blockade did not alter the response to hypoxia, we conclude that leukotrienes are not obligatory mediators of HPV. A critical review of the literature supports a modulatory rather than an obligatory role for leukotrienes in HPV. 