In order to outline the relationship between oxidative phosphorylation, enzyme induction and rat liver regeneration, the effect of one single dose of phenobarbital (PB) on hepatic mitochondria, microsomes and DNA synthesis was investigated. Experiments were performed on intact and partially hepatectomized rats. Results and conclusions can be summarized as follows: (1) While PB has no consistent effect on mitochondrial respiration either in normal or in partially hepatectomized rats, it clearly enhances the rate of DNA synthesis in hepatectomized rats at 24 h. This suggests that the effect of PB on DNA synthesis is independent from the mitochondrial ATP generating activity. (2) PB causes the accumulation of cytochrome P-450 in intact rat hepatocytes, but this effect is suppressed by partial hepatectomy during the first 24 h. This and the above observation on mitochondria suggest that enzyme induction in these experimental conditions is not associated with an increase in mitochondrial oxidative phosphorylation. (3) The mechanism of the influence of PB on DNA synthesis is unclear, but present data suggest that in one single dose PB may behave in two different ways: in intact liver it causes the accumulation of cytochrome P-450 and in partially resected liver it enhances the rate of DNA synthesis; mitochondria playing apparently no important role in this interaction.