The role of nitric oxide (NO) in maintaining homeostasis and regulating organ function during hemorrhagic shock is complex. The inducible NO synthase (iNOS) has been hypothesized to play a critical role in the pathophysiologic consequences of severe hemorrhage. Heat shock protein (HSP) expression is increased by hemorrhage and is a marker of the magnitude of ischemic injury in the liver. HSP induction is protective against injury in animal models of inflammation and is regulated by NO in hepatocytes. To clarify the role of iNOS in hepatic injury and its relationship to HSP expression in hemorrhagic shock, NOS was inhibited with L-N-6-(1-iminoethyl) lysine (L-NIL), which is reported to be a selective inhibitor of the inducible NOS isoform. Doses of 50 &mgr;g/kg or 150 &mgr;g/kg were infused over 1 h at the end of compensated shock. Plasma ornithine carbamoyltransferase (OCT), a specific marker of liver injury, was significantly reduced after hemorrhage with low-dose L-NIL (7.1 ± 1.5 IU/L) compared to saline-treated control rats (13.0 ± 1.5 IU/L,P< 0.005), while high-dose L-NIL significantly increased OCT release (35.9 ± 7.2 IU/L,P< 0.05 versus shock alone) despite a greater MAP after resuscitation. HSP expression (HSP-72 and HSP-32) after hemorrhage was increased by L-NIL treatment at the highest dose. We conclude that excessive NO production from iNOS contributes to shock-induced hepatic injury. Our data suggest HSP expression may reflect the degree of ischemic injury after hemorrhage.