No information is available on perinatal changes in myocardial metabolism in vivo. We measured myocardial supply and flux of fatty acids, carbohydrates, and ketone bodies in chronically instrumented fetal, newborn (1–4 days), and juvenile (7 wk) lambs, by measuring aorta-coronary sinus concentration differences and blood flow. In the fetal lambs, myocardial supply and flux of fatty acids were zero. In the newborn lambs, the supply of fatty acids increased tenfold, but there was no flux of fatty acids. Carbohydrates were the major energy source in fetal and newborn lambs, accounting for 89 and 69% of myocardial oxygen consumption, respectively. In the juvenile lambs, the flux of fatty acids was increased threefold. The supply and flux of carbohydrates were decreased (by 31 and 82%, respectively). The supply and flux of ketone bodies gradually increased with age. We show that the myocardium of the lamb in vivo does not switch immediately after birth from carbohydrates to fatty acids. The mechanisms involved in the development of myocardial fatty acid oxidation remain to be elucidated.