The activity of erythrocyte pyruvate kinase (PK), one of the most important enzymes in the anaerobic glycolytic pathway, was measured in a total of 57 healthy (22 term and 35 preterm) infants. The mean PK activity was 8.98 +/- 3.43 IU/gHb in ten and 16.56 +/- 7.26 IU/gHb in preterm infants. The mean PK activity was significantly higher in preterm babies than ten infants (16.56 +/- 7.26 IU/gHb and 8.98 +/- 3.43 IU/gHb, respectively) (P < .001). A significant negative correlation was found between gestational age, birth weight, and PK activity (r = 0.40, P < .05; r = -0.37, P < .05). No correlation was found between postnatal age and PK activity in both preterm and term infants. The increased PK activity in preterm babies was thought to be due to increased glycolytic activity and energy production in these infants.