Insulin, glucagon, and insulin/glucagon mixtures have been infused for four days into the left portal vein of dogs after portacaval shunt. In the left but not in the right liver lobes, insulin alone reduced atrophy, preserved hepatocyte ultrastructure, and trebled cell renewal. Glucagon alone had no effect. In small doses, glucagon did not potentiate the action of insulin and in large doses it may have reduced the insulin benefit. These studies explain the development of the previously mysterious Eck fistula syndrome, provide clues about in-vivo cell growth control by hormones, and suggests new lines of inquiry about the pathogenesis and/or treatment of several human disease processes.