To clarify the functional adaptability of mitochondria in the regenerating liver, the concentrations of respiratory components, hepatic energy charge levels, cytochrome oxidase activity, and phosphorylative activity were studied in mitochondria obtained from regenerating liver of rats treated with chloramphenicol (CAP). In the hepatectomized groups with CAP treatment, a dose-dependent decrease occurred in the concentrations of cytochrome a(+ a3), cytochrome b, cytochrome c + c1, flavoprotein, and pyridine nucleotide. Cytochrome oxidase activity and phosphorylative activity per milligram of mitochondrial protein also decreased dose dependently in CAP-treated hepatectomized groups, with a significant increase in these values per unit of cytochrome a(+ a3). Hepatic energy charge levels significantly decreased in the hepatectomized groups. However, no significant differences were seen among the hepatectomized groups. However, no significant differences were seen among the hepatectomized groups with or without CAP treatment. Our results suggest that hepatic energy charge is maintained at the same relative level by a compensatory enhancement in phosphorylative capacity associated with cytochrome oxidase activity per unit of cytochrome a(+ a3), in spite of a remarkable decrease in the concentrations of respiratory components.