Myocardial metabolism seems to be markedly abnormal during the first hours of reperfusion after aortic crossclamping. Thus we previously demonstrated no uptake of carbohydrate or lipid substrates 1 hour after coronary operations. Amino acids were the only exogenous substrates taken up by the heart. The aim of the present study was to examine if this metabolic abnormality persisted a few hours later. This was done by measuring coronary sinus blood flow and arterial-coronary sinus differences of oxygen, glucose, free fatty acids, glycerol, lactate, beta-OH-butyrate, and amino acids in a similar group of 19 patients 4 to 5 hours after coronary operations. The results demonstrate a change toward normalization of myocardial free fatty acid use, although the threshold for free fatty acid uptake seemed elevated in comparison with that in the normal postabsorptive state. No correlation was found between free fatty acid uptake and myocardial oxygen consumption. Despite elevated arterial levels of glucose, lactate, pyruvate, and beta-OH-butyrate, no uptake was observed. Myocardial amino acid exchange demonstrated a pattern suggestive of postischemic metabolic adaptation. Several amino acids were extracted, glutamate and branched chain amino acids being the quantitatively most important. The uptake of glutamate and branched chain amino acids correlated with myocardial oxygen consumption, which suggests a direct link to myocardial energy metabolism. Myocardial glutamate uptake seemed to be limited by substrate availability.