The L- and M2-type pyruvate kinase from the liver of 1-day old rats demonstrated no significant activation nor inhibition by treatment with cyclic AMP, glucagon or insulin. Neither was there any change in their isozymic composition. By means of incorporation with [3H]leucine followed by immunoprecipitation, the rates of synthesis of both the L- and M2-type pyruvate kinase were not considerably affected by all three modulators. Insulin and glucagon do not direct an immediate change in the synthesis of liver pyruvate kinase and a fluctuation in the insulin/glucagon ratio is not a probable signal for regulating the isozymic expression in the neonatal period.