Whether tirilazad mesylate (U-74006F) protects against liver injury by inhibition of lipid peroxidation or by cell membrane stabilization was investigated. In male Fischer rats subjected to 20 min of hepatic ischemia followed by reperfusion and injection of 0.5 mg/kg Salmonella enteritidis endotoxin, developed of liver injury was accompanied by lipid peroxidation, as indicated by 81 to 184% increases in hepatic 8-, 9-, 11-, and 12-hydroxyeicosatetraenoic acid content and a 85% increase of plasma F2-isoprostane concentrations at 4 h of reperfusion. Treatment with U-74006F (two bolus doses of 3 mg/kg each; the first dose was injected i.v. 30 min before ischemia and the second dose, at the time of reflow) reduced hepatic injury by 60% but had no significant effect on either parameter of lipid peroxidation. In contrast, U-74006F treatment attenuated liver injury and lipid peroxidation at 24 h reperfusion. Pretreatment with U-74006F in vivo had no effect on lipid peroxidation and liver injury in vitro during perfusion with tert-butylhydroperoxide. However, U-74006F protected hepatocytes significantly against membrane damage induced by cell swelling due to perfusion with hypotonic medium or ischemia-reperfusion. These data support the conclusion that U-74006F enhances the resistance of liver cell membranes to injury by its membrane-stabilizing effect and not by directly scavenging free radicals in vivo. However, the cytoprotective effect of U-74006F can under certain circumstances inhibit recruitment and activation of inflammatory cells, which will then reduce the oxidant stress and lipid peroxidation in the liver.