At birth, hepatic 'active' and 'dichloracetate-activated' pyruvate dehydrogenase complex activities in the newborn of normal, mildly diabetic, and severely diabetic rats were similar. The 'active' and 'dichloracetate-activated' pyruvate dehydrogenase complex activities increased significantly during the first 2 and 6 postnatal h, respectively in the three groups of neonates (p less than 0.05). The greatest increase in both 'active' and 'dichloroacetate-activated' pyruvate dehydrogenase complex activity was observed in the neonates of mildly diabetic rats. Administration of glucose or insulin at birth to the newborn of normal rats caused a significant increase in the percentage of 'active' pyruvate dehydrogenase complex activity within 1 h (p less than 0.01). Similar treatment caused no significant increases in the newborn of severely diabetic rats. The transient increases in 'active' pyruvate dehydrogenase complex activity in the neonates of normal and diabetic rats were consistent with rapid disappearance of blood lactate during the first hours of postnatal life.