Recent studies have suggested an important role for cytochrome P450 enzymes in oxygen - induced tissue injury. In the present study, adult male Sprague-Dawley rats were exposed to hyperoxia (>95% O2) for 48 or 60 h and liver microsomal ethoxyresorufin O-deethylase (EROD) and methoxyresorufin O-demethylase (MROD) activities were determined as measures of CYP1A1 and CYP1A2, respectively. The levels of the apoproteins were assessed by Western analyses. At 48 h, EROD and MROD activities were elevated in the hyperoxia-exposed animals by 7.6-fold and 3.3-fold, respectively, over the corresponding activities in air-breathing controls. These increases were paralleled by increased apoprotein levels of CYP1A1 and CYP1A2 in the hyperoxic animals. At 60 h, the EROD and MROD activities declined to control levels, as did the apoprotein contents of CYP1A1 and CYP1A2. Glutathione peroxidase activities towards cumene hydroperoxide also were measured in the cytosolic fractions. The reduction of cumene hydroperoxide is catalyzed preferentially by glutathione S-transferase Ya subunit, an enzyme encoded by the Ah gene battery. GP activities toward cumene hydroperoxide were not altered significantly in the hyperoxia-exposed animals at either time point. The results indicate that 48 h of hyperoxia induces CYP1A1 to a greater extent than CYP1A2, in analogy to induction by the potent carcinogen 3-methylcholanthrene. However, the lack of induction by hyperoxia of the GP activities toward cumene hydroperoxide implies mechanisms of induction of CYP1A1/1A2 other than via an Ah receptor-mediated mechanism, in contrast with recent proposals regarding induction of CYP1A1/1A2 by hyperoxia. The dramatic decline of CYP1A1/1A2 enzyme activities and protein levels between 48 and 60 h of O2 indicates a surprisingly selective and sudden effect, suggesting that these enzymes may be significant sources of reactive oxygen species at this juncture during prolonged hyperoxic exposure. Supported by NIH GM44263 and HD27823.