Recent reports by us and others have demonstrated an antioxidant role for heme oxygenase (HO) in antioxidant defense(1,2). In its reaction, HO degrades heme, a potential oxidant, and forms bilirubin, a molecule known for its antioxidant properties. Therefore, the sum of the reaction of HO could serve in providing protection against oxygen radical mediated injury. Transgenic animals which lack the constitutive isoenzyme, HO-2 (KO), were developed by Poss et al(3). These animals do not show any morphologic differences compared to wild type(WT) litter mates. We chose to investigate whether the KO animals incur more lung and serum oxidative injury as a result of a 72 hour exposure to >95% O2 than do WT. We measured lung lipid peroxidation, protein oxidation and glutathione content as well as serum lipid hydroperoxides and serum glutathione levels in the KO and WT animals. After 72 hours of hyperoxic exposure lungs of KO had significant increase in lipid peroxidation(1.4-fold), protein oxidation, and significant glutathione depletion (50% of WT) compared to WT. Furthermore, serum lipid hydroperoxides were elevated(1.5-fold) in the KO mutants compared to WT. Northern analysis revealed increased induction of lung HO-1 mRNA in the KO animals suggesting that HO-1 induction occured as a result of increased oxidative stress. We conclude that deficiency of the constitutive form of HO is associated with increased susceptibility to oxidative injury, despite increased expression of the inducible HO-1 mRNA. We speculate that HO is important in antioxidant defense and that there may be differences in function of the constitutive versus the inducible form of this enzyme.