We examined the effects of prolonged hyperoxia (75% O₂) on lung structure and collagen metabolism in the subacute phase of lung injury induced by continuous infusion of endotoxin (LPS) in a rat model. Experimental groups included control, endotoxin alone, endotoxin + hyperoxia, and hyperoxia alone. Endotoxin-treated rats received a bolus of LPS, 10-mg/kg i.v., followed by 500 µg/kg/day in continuous infusion for 10 days. The bronchoalveolar lavage (BAL) fluid/plasma albumin concentration ratio, an index of capillary permeability, and neutrophil and macrophage counts in BAL fluid were highest in the endotoxin + hyperoxia group. On pathologic examination, prolonged hyperoxia exacerbated the destruction of the alveolar wall and caused most prominent emphysematous changes in the endotoxin + hyperoxia group. Lung tissue hydroxyproline concentration was significantly decreased in the hyperoxia group, and increased in the endotoxin group. The latent forms of MMP-2 and MMP-9 increased in the BAL fluid of the endotoxin- and/or hyperoxia-treated groups, while the activities of collagenase and gelatinase, and the active form of MMP-2 were all increased in the hyperoxia-treated degraded collagen, the major structural component of basement membranes, and caused emphysematous changes associated with activation of collagenase and MMP-2. Our observations suggest that, in the subacute phase of endotoxin-induced lung injury, prolonged hyperoxia causes pulmonary emphysematous changes with persistent injury to the alveolar capillary barrier. Collagenase and MMP-2 activated by hyperoxia, together with MMP-9, may play prominent roles in the disruption of the alveolar basement membranes and degradation of collagen lining the alveolar walls.