The activities of ATP-synthetase and ATPase in rabbit liver were evaluated by kinetic analysis of rapid changes in ATP during alteration of oxygen supply, and were compared between normal and hepatectomized rabbits. Velocity constants of ATP-synthetase and ATPase were computed on the assumption that ATP-synthetase and ATPase follow the pseudo-first order reaction of ADP and ATP, respectively. The velocity constant of ATPase increased from the control value of 0.21 to 0.48 (min–1) in the remnant rabbit liver at 24 h after 70% hepatectomy. The velocity constant of ATP-synthetase increased from the control value of 4.05 to 5.24 (min–1) after the hepatectomy. These results indicate that both ATP-synthetase and ATPase are accelerated due to liver regenerat