Arginine synthetase activity and ammonia removal in liver slices were determined in rats with obstructive jaundice or acute carbon tetrachloride (CCl₄)-induced liver dysfunction and the following results were obtained: (1) Urea synthesis and ammonia removal in liver slices progressively decreased with prolonged biliary obstruction. The effects of ATP and/or ornithine addition were also markedly decreased, particularly in the group with 6 weeks of biliary obstruction. (2) Arginine synthetase activity also fell with prolongation of biliary obstruction and the fall was most pronounced in the 6-week group. (3) The CCl₄-induced liver dysfunction group showed a significantly higher level of arginine synthetase activity than the group with 6 weeks of biliary obstruction, but ammonia removal was markedly decreased and the effects of ATP and ornithine addition were prominent. From the results, it is concluded that, due to a fall in the enzyme activity of the urea cycle in obstructive jaundice, liver dysfunction can easily occur with prolongation of the obstruction, while hepatic urea cycle dysfunction is brought about by severe metabolic disruption in the liver damaged by CCl₄.