Reactive oxygen intermediates have been implicated in lung injury induced by inhaled irritants. The present studies utilized mice overexpressing Cu/Zn-superoxide dismutase (SOD+/+) to analyze their role in ozone-induced lung inflammation and cytotoxicity. Treatment of wild type (WT) mice with ozone (0.8 ppm, 3 h) resulted in increased bronchoalveolar lavage fluid (BALF) protein, which was maximal after 24-48 h. Significant increases in lung macrophages and 4-hydroxyalkenals (4-HAK) were also observed. In contrast, BALF protein and macrophage content and 4-HAK were at control levels in ozone treated SOD+/+ mice. There was also no evidence of peroxynitrite-mediated lung damage demonstrating that SOD+/+ mice are resistant to ozone toxicity. Whereas alveolar macrophages from WT mice produced increased amounts of nitric oxide and expressed more inducible nitric oxide synthase, phospholipase A₂ and tumor necrosis factor-alpha after ozone inhalation, this was not evident in cells from SOD+/+ mice. Ozone-induced decreases in interleukin-10 were also not observed. In WT mice ozone inhalation resulted in activation of NF-B, which regulates proinflammatory gene activity. This response was significantly reduced in SOD+/+ mice. These data demonstrate that antioxidant enzymes play a critical role in ozone-induced tissue injury and in inflammatory mediator production.