Hepatotoxic doses of acetaminophen to mice produce not only acetaminophen-protein adducts in the centrilobular cells of the liver, but nitrotyrosine-protein adducts in the same cells, the site of the necrosis. Nitration of tyrosine occurs with peroxynitrite, a species formed by reaction of nitric oxide (NO·) with superoxide (O2·−). Because NO· and O2·− may be produced by activated Kupffer cells and/or infiltrated macrophages, we pretreated mice with the macrophage inactivators/depeleters gadolinium chloride (7 mg/kg, intravenously [iv]) or dextran sulfate (10 mg/kg, iv) 24 hours before administration of acetaminophen (300 mg/kg). Mice treated with acetaminophen plus gadolinium chloride, or acetaminophen plus dextran sulfate, had significantly less evidence of hepatotoxicity as evidenced by lower serum alanine transaminase (ALT) levels (28 ± 1 IU/L and 770 ± 240 IU/L, respectively) at 8 hours compared with acetaminophen (6,380 ± 408 IU/L). Analysis of hepatic homogenates for acetaminophen-protein adducts at 2 hours, a time of maximal covalent binding and before hepatocyte lysis, indicated that these pretreatments did not decrease covalent binding. Western blot analysis for the macrophage marker protein F4/80 in homogenates revealed not only the expected decrease by the macrophage inactivators/depleters, but also an apparent increase in acetaminophen-only–treated mice. At 8 hours nitrotyrosine-protein adducts were present in the acetaminophen-only–treated mice, but not in the acetaminophen plus gadolinium chloride–treated mice, or acetaminophen plus dextran sulfate–treated mice. High levels of heme-protein adducts, a measure of oxidative stress, were detected in livers of the 8 hour acetaminophen-only–treated mice. These data suggest that acetaminophen hepatotoxicity is mediated by an initial metabolic activation and covalent binding, and subsequent activation of macrophages to form O2·−, NO·, and peroxynitrite. Nitration of tyrosine correlates with toxicity.