Children of mothers with insulin-dependent diabetic mothers (IDDM) have a 2-4 times higher incidence of congenital birth defects as compared to the general population, including cardiac abnormalities, of unknown etiology. Using rodent embryos to explore potential teratogenic factors of the altered IDDM metabolism, it has been shown that exposure to hypoglycemia in vitro results in a variety of defects, including cardiac malformations. Since pregnant diabetics experience frequent episodes of low blood glucose, it was hypothesized that hypoglycemia may play a role in the generation of heart abnormalities seen in children born to IDDM mothers. Several studies have indicated that during embryogenesis the heart is dependent on glucose for energy production such that under hypoglycemic conditions, insufficient amounts of ATP may be produced resulting in abnormalities. To test this hypothesis, cardiac ATP content was monitored in D10-D12 (plug day = D1) hearts. In addition, the contribution of glycolysis and the Krebs cycle to ATP production was monitored. D10 hearts exposed to euglycemic control conditions were found to be primarily dependent on glycolysis for ATP production from glucose before switching to the Krebs cycle and oxidative phosphorylation for energy production from this substrate on D11. Exposure to hypoglycemia did not alter the timing of this maturation process or deplete cardiac ATP content. However, cardiac lactate levels increased approximately twofold in the presence of hypoglycemia on d10. Since increased concentrations of lactate are harmful to many tissues and have been shown to be detrimental to the adult rat heart, lactic acidosis may explain the origin of cardiac defects produced by hypoglycemia, and not a deficiency of ATP.