Peroxynitrite may be involved in acetaminophen-induced liver damage. However, it is unclear if peroxynitrite is generated in hepatocytes or in the vasculature. To address this question, we treated C3Heb/FeJ mice with 300 mg/kg acetaminophen and assessed nitrotyrosine protein adducts as indicator for peroxynitrite formation. Vascular nitrotyrosine staining was evident before liver injury between 0.5 and 2 h after acetaminophen treatment. However, liver injury developed parallel to hepatocellular nitrotyrosine staining between 2 and 6 h after acetaminophen. The mitochondrial content of glutathione disulfide, as indicator of reactive oxygen formation determined 6 h after acetaminophen, increased from 2.8 +/- 0.6% in controls to 23.5 +/- 5.1%. A high dose of allopurinol (100 mg/kg) strongly attenuated acetaminophen protein-adduct formation and prevented the mitochondrial oxidant stress and liver injury after acetaminophen. Lower doses of allopurinol, which are equally effective in inhibiting xanthine oxidase, were not protective and had no effect on nitrotyrosine staining and acetaminophen protein adduct formation. In vitro experiments showed that allopurinol is not a direct scavenger of peroxynitrite. We conclude that there is vascular peroxynitrite formation during the first 2 h after acetaminophen treatment. On the other hand, reactive metabolites of acetaminophen bind to intracellular proteins and cause mitochondrial dysfunction and superoxide formation. Mitochondrial superoxide reacts with nitric oxide to form peroxynitrite, which is responsible for intracellular protein nitration. The pathophysiological relevance of vascular peroxynitrite for hepatocellular peroxynitrite formation and liver injury remains to be established.