To examine the role of nitric oxide (NO) in liver ischemia-reperfusion injuries, electron spin resonance (ESR) measurements were employed to directly detect NO in the liver and blood of mice subjected to liver ischemia-reperfusion. We used an NO trapping and accumulating reagent, N- (dithiocarboxy)sarcosine/Fe2+ (DTCS/Fe) that specifically binds to NO to form stable NO adduct, (DTCS)/Fe-NO. The DTCS/Fe-NO adduct is detectable by ESR spectrometry at room temperature, giving rise to a characteristic triplet spectrum. The (DTCS)/Fe-NO contents in liver and blood decreased during ischemia, and still more decreased at the early reflow phase. Then, 12 h after reflow onset, the adduct content increased approximately 1.5-fold in the liver and 2.5-fold in the blood compared to that in a non-ischemia group. The fluctuation of liver NO levels may have resulted from a lack of oxygen during ischemia and the scavenging of superoxide anions during the early reflow phase. Therefore, NO levels would result in decline, then a subsequent boost in NO levels as cells respond to the reperfusion-induced damage. Blood endotoxin concentration and serum aspartate oxoglutarate aminotransferase (AST), alanine oxoglutarate aminotransferase (ALT), and lactate dehydrogenase (LDH) levels were significantly higher in the liver ischemia-reperfused group than in the sham-operated group. Pretreatment with N(G)-mono-N-methyl-L- arginine, an NO synthase inhibitor resulted in decreased NO levels to an undetectable level, all the more increased lipid peroxide contents in liver, and the amelioration of hepatic injury induced by 60 min of liver ischemia followed by 60 min of reperfusion. From these results, NO was interpreted to protect against injuries in part to brake the progression of the free radical chain reaction.