The present study was undertaken to determine whether CoQ10 pretreatment could modify cellular free radical metabolism in regenerating rat liver and enhance hepatic DNA synthesis. Male Wistar rats (250-300 g) were used. Partial 70% hepatectomy was performed according to Higgins and Anderson. CoQ10 (6 mg/kg body weight) and its solvent as a placebo were injected intravenously 1 hour before operation. Twenty-four hours or 48 hours after 70% hepatectomy resulted in 55% or 50% decreases of hepatic ATP, and marked decreases of endogenous CoQ homologs (CoQ9 and CoQ10), alpha-tocopherol and reduced glutathione (GSH) in remnant liver and was accompanied by 2.5-fold increases in lipid peroxides. However, in CoQ10-treated animals synthesis of ATP was accelerated after 48 hours after partial hepatectomy. There were no changes in the levels of CoQ9, alpha-tocopherol, or GSH or in the level of the enhanced CoQ10 in regenerating liver. The pretreatment also completely suppressed the elevation of lipid peroxide. Moreover, CoQ10 administration markedly increased hepatic DNA synthesis at 48 hours after partial hepatectomy. These results indicate that cellular damage during hepatic regeneration is in part due to lipid peroxidation, and suggest that CoQ10 is effective in the pretreatment of liver resection.