1. The L and M1 isozymes of pyruvate kinase were purified to homogeneity from rat liver and muscle respectively and their specific antibodies were employed to quantify the isozyme concentration in rat liver during development. 2. Total enzyme activity decreases towards birth and reaches a minimum on the 3rd postnatal day, but the activity increases dramatically after weaning. 3. Immunoprecipitation revealed that the M2 type predominates in the prenatal period but decreases sharply just before birth. 4. The L isozyme contribution is augmented upon weaning and is sustained until the rat is adult and a L/M ratio of 9:1 is maintained. 5. By means of incorporation studies with [3H]leucine followed by immunoprecipitation, the increase in L-type activity when approaching term and after weaning is explained by a twofold increase in its rate of synthesis coupled with a concomitant reduction of the M2-type synthesis.