Abstract The mechanism and sequence of events of the induction of liver pyruvate kinase by sucrose and fructose administration were examined. Actinomycin and cycloheximide prevented the sucrose and fructose induced rise in pyruvate kinase activity. In addition to ADP, GDP, UDP, IDP and, to a minor extent, CDP and TTP were able to function as acceptors for pyruvate kinase for both liver and hepatoma. At saturating ADP levels ATP, GTP, UTP, ITP, and, to a minor extent, CTP were inhibitors of liver pyruvate kinase activity. GTP was a stronger inhibitor for hepatoma pyruvate kinase whereas UTP was not inhibitory. At low ADP levels TTP was a good inhibitor and the other nucleotides are being studied. l -Alanine was a strong competitive inhibitor of liver pyruvate kinase and of the hepatoma enzyme; however, it had little effect on the muscle or brain enzyme. In contrast, l -phenylalanine was a good inhibitor of rat and human brain pyruvate kinase. It is suggested that the inhibition of human adult and fetal brain pyruvate kinase by l -phenylalanine may have a role in the metabolic damage in phenylketonuria. The chief mechanisms governing the rate and direction of the opposing metabolic pathways of gluconeogenesis and glycolysis in the liver were discussed.