SummaryInhibition of prostaglandin cyclooxygenase augments hypoxic pulmonary vasoconstriction. We used a neonatal lamb lung preparation perfused with Krebs' bicarbonate buffer to characterize and quantify prostanoids produced by the pulmonary vasculature from endogenous arachidonic acid in the absence of formed blood elements during ventilation with normoxic and hypoxic gas mixtures. Prostaglandin (PG) I2synthesis increased from 6.4 ± 2.7 ng/min (SEM) during normoxic ventilation to 14.3 ± 5.4 ng/min during hypoxia and returned to 4.7 ± 1.2 ng/min with resumption of normoxia. These data demonstrate that hypoxia stimulates pulmonary vascular synthesis of prostaglandin I2from endogenous substrate in neonatal lambs and suggest that the augmentation of hypoxic pulmonary vasoconstriction by prostaglandin cyclooxygenase inhibition is due, at least in part, to interference with the synthesis of this vasodilator prostanoid.