Macrophage-derived inflammatory mediators have been implicated in tissue injury induced by a number of hepatotoxicants. In the present studies, we used transgenic mice with a targeted disruption of the gene for inducible nitric oxide synthase (NOS II) to analyze the role of nitric oxide in inflammatory mediator production in the liver and in tissue injury induced by acetaminophen. Treatment of wild-type mice with acetaminophen (300 mg/kg) resulted in centrilobular hepatic necrosis, which was evident within 3 h and reached a maximum at 18 h. This was correlated with NOS II expression and nitrotyrosine staining of the liver, which was most prominent after 6 h. Expression of mRNA for tumor necrosis factor-alpha (TNF-alpha), interleukin-10 (IL-10), matrix metalloproteinase-9, and connective tissue growth factor (CTGF) also increased in the liver following acetaminophen treatment of wild-type mice. NOS II knockout mice were found to be less sensitive to the hepatotoxic effects of acetaminophen than wild-type mice. This did not appear to be due to differences in acetaminophen-induced glutathione depletion or adduct formation. In NOS II knockout mice treated with acetaminophen, hepatic expression of TNF-alpha, as well as CTGF, was significantly increased compared to wild-type mice. In contrast, IL-10 expression was reduced. These data demonstrate that nitric oxide is important in hepatotoxicity induced by acetaminophen. Moreover, some of its effects may be mediated by altering production of pro- and antiinflammatory cytokines and proteins important in tissue repair.