How higher organisms respond to elevated oxidative stress in vivo is poorly understood. Therefore, we measured oxidative stress parameters and gene expression alterations (Affymetrix arrays) in the liver caused by elevated reactive oxygen species induced in vivo by diquat or by genetic ablation of the major antioxidant enzymes, CuZn-Superoxide Dismutase (Sod1) and Glutathione Peroxidase-1 (Gpx1). Diquat (50 mg/kg) treatment resulted in a significant increase in oxidative damage within 3 to 6 hours in wild type mice without any lethality. In contrast, treating Sod1^-/- or Gpx1^-/- mice with a similar concentration of diquat resulted in a significant increase in oxidative damage within an hour of treatment and was lethal, i.e., these mice are extremely sensitive to the oxidative stress generated by diquat. The expression response to elevated oxidative stress in vivo does not involve an upregulation of classical antioxidant genes, though long-term oxidative stress in the Sod1^-/- mice leads to a significant upregulation of thiol antioxidants (e.g., Mt1, Srxn1, Gclc, Txnrd1), which appears to be mediated by the redox-sensitive transcription factor, Nrf2. The main finding of our study is that the common response to elevated oxidative stress, with diquat treatment in wild type, Gpx1^-/-, Sod1^-/- mice and in untreated Sod1^-/- mice, is an upregulation of p53 target genes (p21, Gdf15, Plk3, Atf3, Trp53inp1, Ddit4, Gadd45a, Btg2, Ndrg1). A retrospective comparison with previous studies shows that induction of these p53-target genes is a conserved expression response to oxidative stress, in vivo and in vitro, in different species and different cells/organs.