Matrix metalloprotease-9 (MMP-9) is increased in lung injury following hyperoxia exposure in neonatal mice, in association with impaired alveolar development. We studied the role of MMP-9 in the mechanism of hyperoxia-induced functional and histological changes in neonatal mouse lung. Significance. Reduced alveolarization with remodeling of extracellular matrix (ECM) is a major morbidity component of oxidant injury in developing lung. Hypothesis. Matrix metalloprotease-9 mediates oxidant injury in developing lung causing altered lung remodeling. Methods. 5-day-old neonatal wild type (WT) and MMP-9 (-/-) mice were exposed to hyperoxia for 8 days. The lungs were inflation fixed and sections were examined for morphometry. The mean linear intercept and alveolar counts were evaluated. Immunohistochemistry for MMP-9 and elastin was performed. MMP-2, MMP-9, Type I collagen and tropoelastin were measured by Western blot analysis. Lung quasi static compliance was studied in anaesthetized mice. Results. MMP-2 and MMP-9 were significantly increased in lungs of WT mice exposed to hyperoxia compared to controls. Immunohistochemistry showed an increase in MMP-9 in mesenchyme and alveolar epithelium of hyperoxic lungs. The lungs of hyperoxia-exposed WT mice had less gas exchange surface area and were less compliant compared to room air-exposed WT and hyperoxia-exposed MMP-9 (-/-) mice. Type I collagen and tropoelastin were increased in hyperoxia exposed WT with aberrant elastin staining. These changes were ameliorated in hyperoxia-exposed MMP-9 (-/-) mice. Conclusion. MMP-9 plays an important role in the structural changes consequent to oxygen-induced lung injury. Blocking MMP-9 activity may lead to novel therapeutic approaches in preventing BPD.