The changes in the contents of the main tricarboxylic acid cycle intermediates and related amino acids under total ischemia and subsequent reperfusion of isolated guinea pig heart were studied. The decrease in ATP and phosphocreatine during 30 min ischemia was accompanied by alanine formation and approximately stoichiometric glutamate loss. The increase in malate in ischemic myocardium corresponded to the anaplerotic flux aspartate----oxaloacetate----malate. The succinate production was commensurable to alpha-ketoglutarate formation in the alanine aminotransferase reaction. The release of bulk amount of lactate, alanine and succinate into the myocardial effluent was observed during an early phase of the reperfusion using 1H NMR. In contrast to these metabolites, malate release was not observed in the reperfusion. By the 30th min of the reperfusion the decrease in lactate, alanine, malate and succinate tissue contents to the preischemic values was accompanied by the recovery of ATP and phosphocreatine. The results suggest that the formation and the release of succinate and alanine from the heart, complementary to that of lactate, reflect profound disturbances in energy metabolism.