We investigated the regulation of antioxidant system under acetaminophen (AAP) toxicity. Twelve male New Zealand rabbits were divided into two groups with the following treatments: Group 1 animals were intraperitoneally injected with single saline (control). Group 2 animals were treated with intraperitoneal injection of AAP at a dose of 250 mg/kg body weight. Four hours following the treatments, blood samples were collected and the rabbits were sacrificed to collect liver samples. Hepatocellular damage was evaluated by aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels as well as histopathological examinations and immunohistochemical analysis. Tissue-reduced glutathione (GSH), nitric oxide (NO(center dot)), and malondialdehyde (MDA) levels were also measured. mRNA expression levels of the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px) were measured by semi-quantitative RT-PCR. It was found that liver GSH was reduced significantly in AAP-treated rabbits (P < 0.05), while MDA and NO(center dot) levels were increased when they were compared to control (P < 0.05). Blood AST and ALT levels were also increased following AAP treatment (P < 0.05). Hepatocellular degeneration and severe necrosis were detected in histopathological examinations. Increased immunostaining was observed for inducible nitric oxide synthase (iNOS) and nitrotyrosine in the liver. There were no changes in mRNA expression levels of SOD, CAT, and GSH-Px after AAP treatment compared to control group. These results suggest that the expression of these enzymes, which are involved in the antioxidant system, may not be altered after AAP toxicity, although classical toxic changes such as depletion of GSH, hepatocellular necrosis, and increased immunostaining for iNOS and nitrotyrosine were detected.