The lipid peroxidation product 4-hydroxynon-2-enal (4-HNE) is a strong electrophile that forms covalent adducts with proteins and, to a lesser extent, nucleic acids and phospholipids. The generation of 4-HNE appears to be an inevitable consequence of aerobic metabolism. The metabolism of 4-HNE is mainly, although not entirely, conjugative, and proceeds via Michael addition of glutathione to the double bond of 4-HNE. This reaction is catalyzed by specialized glutathione S-transferases (GSTs) exemplified by the murine mGSTA4-4. To study the (patho)physiological effects of 4-HNE in an intact organism, we disrupted the mGsta4 gene in the mouse. The resulting mGsta4 null mouse expressed no mGsta4 mRNA and no corresponding protein, had a reduced ability to conjugate 4-HNE, and had an increased steady-state level of this aldehyde in tissues. The residual conjugating activity for 4-HNE (23-64% depending on the tissue) is probably attributable to isoforms of glutathione S-transferases which have low catalytic efficiency for 4-HNE but are more abundant than mGSTA4-4, or are upregulated upon mGsta4 gene disruption. Mice homozygous for the disrupted mGsta4 allele were viable and appeared normal except for lower litter size, higher fat content in bones, and greater susceptibility to bacterial infection. The null mice had a significantly lower survival time than wild-type controls when chronically treated with relatively low doses of paraquat, a finding consistent with a role of mGSTA4-4 in the defense against oxidative stress. The mouse model should be useful for the study of degenerative conditions in which 4-HNE is postulated to be a contributing factor.