The time course of changes in mitochondrial energy metabolism during liver regeneration, following partial hepatectomy, is analyzed. For 24 h after surgical operation, a lag phase in the time course of the growth of liver is observed. In this period mitochondria showed a decrease of: (1) the respiratory control index; (2) the rate of oxidative phosphorylation; (3) the amount of immunodetected beta-F-1 and F(0)1-PVP subunits of F0F1-ATP synthase. No decrease, but instead a small increase in the content of mRNA for beta-F-1 was observed in this phase. After this lag phase the growth of liver started, the content of mRNA for beta F-1, as well as the level of immunodetected mitochondrial beta-F-1 and F(0)1-PVP subunits, increased and oxidative phophorylation recovered. Analysis of the relative beta F-1 protein/mRNA ratio indicates a decrease of beta F-1 translational efficiency which remained low up to 72 h after partial hepatectomy and reached the same ratio of control at 96 h. It is concluded that the regenerating capability of rat liver is correlated with the efficiency of oxidative phosphorylation.