Neonatal chronic lung disease (CLD), caused by prolonged mechanical ventilation (MV) with O₂-rich gas, is the commonest cause of long-term hospitalization and recurrent respiratory illness in extremely premature infants. Recurrent episodes of hypoxemia and associated ventilator adjustments often lead to worsening CLD. The mechanism that causes these hypoxemic episodes is unknown. Hypoxic pulmonary vasoconstriction (HPV), which is partially controlled by O₂-sensitive voltage-gated potassium (Kv) channels, is an important adaptive response to local hypoxia that helps to match perfusion and ventilation in the lung. To test the hypothesis that chronic lung injury (CLI) impairs HPV, we studied preterm lambs that had MV with O₂-rich gas for 3-weeks and newborn rats that breathed 95%-O₂ for 2-weeks, both of which resulted in airspace enlargement and pulmonary vascular changes consistent with CLD. HPV was attenuated in preterm lambs with CLI after 2-weeks of MV and in newborn rats with CLI after 2-weeks of hyperoxia. HPV and constriction to the Kv1.x-specific inhibitor, correolide, were preferentially blunted in excised distal pulmonary arteries (dPAs) from hyperoxic rats, whose dPAs exhibited decreased Kv1.5 and Kv2.1 mRNA and K⁺ current. Intrapulmonary gene transfer of Kv1.5, encoding the ion channel that is thought to trigger HPV, increased O₂-sensitive K⁺ current in cultured smooth muscle cells from rat dPAs, and restored HPV in hyperoxic rats. Reduced expression/activity of O₂-sensitive Kv channels in dPAs contributes to blunted HPV observed in neonatal CLD.