The rate of mitochondrial oxidative phosphorylation and the cytosolic and mitochondrial total and oxidized glutathione concentrations were studied in regenerating rat livers after partial (70%) hepatectomy. The rate of mitochondrial oxidative phosphorylation progressively decreased during the early prereplicative phase of liver regeneration. This was accompanied by a progressive decrease in mitochondrial, but not cytosolic, glutathione concentration. Twenty-four hours after partial hepatectomy, both the rate of adenosine triphosphate (ATP) synthesis and the amount of mitochondrial glutathione were depressed by 50% with respect to controls (sham-operated animals). During the second replicative phase, both the oxidative phosphorylation rate and mitochondrial glutathione concentration were recovered; however, the kinetics of the recovery were different, being the total amount of mitochondrial glutathione completely restored 48 hours after partial hepatectomy, whereas 72 hours were needed for the recovery of oxidative phosphorylation. The decrease in the rate of oxidative phosphorylation, during the early phase of liver regeneration, appeared to be secondary to the decreased content of the catalytic subunit beta-F1 of the ATP synthase complex, which in turn was shown to be linearly related to the decrease of intramitochondrial glutathione. These observations suggest that the two phenomena may be due to the previously reported increased free radical production during the early phase of liver regeneration. The depression of mitochondrial glutathione after partial hepatectomy may play a contributory role in structural and functional alterations of mitochondria occurring in the first retrodifferential phase of liver regeneration.