The activities of key gluconeogenic enzymes in the liver of newborn guinea pigs delivered vaginally at term were monitored as a function of time following birth. The activities of glucose-6-phosphatase and fructose-1,6-diphosphatase did not show a significant increase over the first 72 h of life, neither did the activity of mitochondrial phosphoenolpyruvate carboxykinase. The mitochondrial enzyme pyruvate carboxylase and the cytosolic phosphoenolpyruvate carboxykinase (PEPCK) both increased significantly in the first 24 h postpartum. Mitochondrial protein and succinate dehydrogenase activities showed only slight increases in the 72-hour period. Rapid depletion of liver glycogen was evident in these animals following birth, but severe hypoglycaemia was not evident. Mitochondrial and cytosolic PEPCK showed similar kinetic behaviour with respect to their affinities for oxalacetate and divalent metal cation Mn++, though the mitochondrial enzyme would accept Mg++ as the divalent metal in place of Mn++. The role of the compartmented PEPCK activities is discussed.