The liver expresses a considerable amount of nitric oxide (NO) upon induction with cytokines or/and endotoxin. The NO synthesized by inducible NO synthase (NOS) of the liver see ms to play a role in various hepatic physiological processes. Here we investigate the effects of NO on acetaminophen (AA)-induced liver injury. The treatment of S-nitros-N-acetyl penicillamine (SNAP, exogenous NO donor) at the dose of 0.1mM decreased AA-induced hepatotoxicity suggesting the possibility of NO to play a role in protection from the hepatotoxicity induced by AA. On the other hand, the excessive NO produced by NO donor (SNAP: 0.5, 2.5, 6.25mM) has been shown to cause a concentration dependent hepatotoxicity, and such damages was decreased by Superoxide and increased by superoxide dismutase, indicating that the hepatotoxicity induced by excessive NO depends on balancing between NO and superoxide. Taken together, the results indicate that NO has biphasic effects on hepatotoxicity.