The role of lactate as an energy substrate in fetal (0.9 gestation) and newborn (2 day old) hearts was investigated in isolated, perfused hearts. Perfusions were performed with Krebs-Henseleit buffer supplemented with glucose (5 mM) in combination with varying concentrations of lactate. Isolated working heart perfusions, in which the heart ejects the buffer at controlled pressure, were carried out with glucose (5 mM) alone and with glucose (5 mM) and lactate (5 mM) combined. With glucose as sole substrate, lactate was produced by the heart and glucose uptake accounted for approximately two-thirds of oxygen consumption. When both glucose and lactate were provided, lactate accounted for more than 80% of oxygen consumption and profoundly suppressed glucose uptake. Further investigations using retrograde perfusion through the aorta demonstrated that lactate uptake was consistently observed when exogenous lactate concentrations exceeded 1.25 mM. Glucose uptake was suppressed with lactate concentrations as low as 0.5 mM and progressive suppression occurred with increasing lactate concentrations. Fetal and newborn pig hearts utilize lactate as a primary substrate for energy production when lactate concentrations are in the physiological range.