At present, the physiological role of NO * synthesis in the liver is ambiguous. Studies directed to reveal the role of NO * in relation to liver function were primarily initiated by an interest in the hepatic response to infections and the consequent modulation of liver function. The purpose of the present investigation was to use perfused rat hepatocytes to test the ability of the latter to produce NO * and to delineate the relationship between exogenously delivered NO* and any alteration in the degree of injury as produced by anoxia/reoxygenation (AR) or D-galactosamine (GalN, 5 mM) intoxication. NO * production in rats was stimulated by a single dose of lipopolysaccharide (LPS, 20 mg/kg i.p.) from which hepatocytes were isolated and perfused. Exogenous NO * was delivered to the perfusate of hepatocytes that were isolated from untreated rats, by the addition of sodium nitroprusside (SNP, 2 mM and 0.2 mM). AR and GalN hepatocyte injury was followed after the addition of SNP. Rat hepatocytes were immobilized in low-gelling agarose and perfused with Williams E medium. Endogenous synthesis of NO ’ and exogenous NO * as produced by SNP was evaluated by estimating the end products of NO * (N02" + N03“) in the perfusion medium. The functional and structural integrity of hepatocytes was evaluated from lactate dehydrogenase (LD) leakage and urea synthesis in the perfusion medium. Normal, AR- and GalN-injured hepatocytes did not exhibit measurable NO * while LPS-treated hepatocytes produced NO * (80 //M N02_ + N03_). SNP-produced NO * significantly increased or decreased LD leakage in AR at 2 mM or 0.2 mM, respectively, and also reduced or increased the rate of urea synthesis, respectively. 0.2 mM SNP increased trypan blue exclusion by hepatocytes. On the other hand, GalN toxicity was not significantly altered by SNP as demonstrated by LD leakage and the rate of urea synthesis was increased by SNP addition. The present data suggest both deleterious and beneficial role of NO * in AR liver injury model depending on the level of NO * generated.