We have studied the postnatal ontogeny of creatine kinase (CK) and the glycolytic enzymes phosphoglycerate kinase (PGK), phosphoglycerate mutase (PGM), enolase (En), and pyruvate kinase (PK) in rat brain and uterus. In 30-day-old rat, brain and uterus express the fetal isoforms CK-B, PGK-A, PGM-B, En-alpha and PK-M2, and the differentiated isoforms En-gamma and PK-M1. The activity of glycolytic enzymes in uterus of two-day-old rat is as in brain, while CK activity is 3 times higher in brain. The activity of the glycolytic enzymes in brain began to increase (3-4-fold) 10 days after birth, in a coordinated manner. CK activity began to increase 5 days after birth in both brain (4.2-fold) and uterus (4.5-fold), suggesting the dissociation of glycolytic enzyme ontogeny from CK. In contrast to brain, the levels of glycolytic enzymes in uterus were highest at birth, suggesting the action of a tissue-specific mechanism for regulation of the constitutive levels of glycolytic isozymes. Except for PGM, all enzymes showed an increase in total activity, in response to estrogen, in uterus but not in whole brain.