The relationship between acetaminophen (APAP) reactive metabolite formation, nitrotyrosine (NT) production, and cytokine elevation in APAP toxicity was investigated. Mice were dosed with 300 mg/kg of APAP and sacrificed at 1, 2, 4, 8, and 12 h. Serum aspartate aminotransferase (AST) was elevated by 4 h. The relative amount of NT correlated with toxicity and was localized in the necrotic cells. IL-1b was increased at 1 h, whereas IL-6, MIP-2, and MCP-1 were increased by 4-8 h. To determine the importance of reversible versus toxic events, N-acetylcysteine (NAC) was administered to mice either before APAP or 1, 2, or 4 h after APAP. The animals were sacrificed at 12 h. NAC treatment before APAP resulted in serum AST, serum nitrate plus nitrite as a measure of nitric oxide (NO) production, and hepatic cytokine levels that were similar to the controls. No APAP protein adducts or NT was present in these animals. In mice treated with NAC at 1 h, cytokines and serum AST were normal at 12 h, but APAP protein adducts were present in the hepatic centrilobular areas. No NT was present in these animals. In mice treated with NAC at 2 h and sacrificed at 12 h, serum AST was reduced by 80%. APAP adducts and NT were present in the centrilobular areas. Mice receiving NAC at 4 h had no protection from toxicity and serum nitrate plus nitrite. The NT and cytokine levels were similar to those of mice receiving APAP alone. The data suggest a relationship between metabolic events in APAP toxicity and the upregulation of NO, and IL-1b. IL-6, MIP-2, and MCP-1 appear to follow the toxicity. While it is a pre-requisite event, covalent binding per se does not appear to be a toxic event in the development of toxicity.