Changes in the ratio of acetoacetate to 3-hydroxybutyrate (the ketone body ratio) in arterial blood were investigated after massive liver resection in patients and rabbits. In a patient who had had a successful hepatectomy, the blood ketone body ratio decreased rapidly after the operation and then returned to the preoperative level; whereas in a patient who had had an unsuccessful hepatectomy, the blood ketone body ratio did not decrease rapidly after the operation. In 70% hepatectomized rabbits, the blood ketone body ratio decreased maximally nine hours after hepatectomy and then returned gradually to a normal level. The blood ketone body ratio paralleled the hepatic energy charge after 70% hepatectomy in rabbits. The measurement of the blood ketone body ratio is very useful for evaluation of the energy status of the remnant liver.