We have determined eicosanoid production from endogenous arachidonic acid by neonatal lamb lungs stimulated with calcium ionophore A23187 during normoxia and hypoxia. Lungs of lambs 19 to 25 d of age were isolated and perfused with cell-free Krebs' bicarbonate buffer at a flow rate of 15 to 20 ml/kg/min. After 30 min of equilibration in a recirculating system, A23187 was added to the perfusate in a 5-µM concentration and perfusion continued for 15 min more. Eicosanoids were measured in perfu|sate and lung homogenate supernatant. Cyclooxygenase metabolites prostaglandin (PG) E₂, thromboxane A₂, and PGI₂ were measured by radioimmunoassay, and 5-lipoxygenase metabolites leukotrienes (LT) B₄, C₄, D₄, and E₄ by high performance liquid chromatography. During normoxia, all three cyclooxygenase metabolites were present in perfusate, but only PGI₂ and thromboxane A₂ were present in lung homogenate supernatant. Prostacyclin constituted 50% of all the cyclooxygenase products measured. LTC₄ and LTD₄ were detected in both perfusate and lung homogenate supernatant with little production of LTE₄ and LTB₄. During hypoxia, the profile of cyclooxygenase products was unchanged and prostacyclin production was not increased. However, the profile of leukotriene metabolites was altered. LTC₄ synthesis was markedly reduced. The synthesis of LTE₄ and LTB₄ was increased 10-fold, with most of the leukotrienes being retained in lung tissue. We conclude that hypoxia significantly alters leukotriene metabolism of endogenous arachidonic acid by calcium ionophore-stimulated lungs. The increased production by stimulated lungs during hypoxia of LTE₄, a substance that may increase lung capillary permeability, and that of LTB₄, a powerful chemoattractant, may be important contributing factors to lung injury.