A causative factor in the development of diabetes-induced heart dysfunction may be abnormalities in myocardial energy metabolism. Using13C-NMR spectroscopy, we investigated the effects of experimentally induced diabetes (streptozotocin 65 mg/kg, i.v.) on glucose metabolism and contractile function in the isolated perfused rat heart. Hearts from streptozotocin-treated and untreated control rats were perfused with 11 mM [1-13C]glucose as substrate and1H-decoupled13C-spectra recorded for up to 90 min. Incorporation of label from [1-13C]glucose into lactate and glutamate was observed in hearts from control animals, consistent with metabolism through glycolysis and TCA cycle, respectively. Diabetic hearts did not incorporate label into lactate or glutamate. Addition of insulin (0.05 U/ml) to the buffer resulted in the appearance of [3-13C]lactate, although glutamate labeling was not observed. Addition of insulin plus dichloroacetate (2 mM) resulted in incorporation of label from [1-13C]glucose into 2-, 3- and 4-13C-glutamate, indicating glucose entry into the TCA cycle. Addition of insulin, or insulin plus dichloroacetate to control hearts did not alter labeling of either lactate or glutamate. Cardiac function in hearts from the diabetic group was depressed compared to controls and declined significantly over the duration of the experiment. These studies show that concomitant with a decrease in cardiac function, glucose oxidation is profoundly inhibited following the induction of diabetes with streptozotocin. These observations are consistent with a combination of decreased glucose transport and a decrease in pyruvate dehydrogenase activity. © 1993 Academic Press Limited.