The human heart in the fasting state extracts FFA, glucose, lactate, pyruvate, and ketone bodies from the systemic circulation. Of these substrates, FFA utilization accounts for the greater part of oxygen consumption and energy production. The oxidative use of lipid (FFA) and carbohydrate (glucose and lactate) fuels is reciprocally regulated through the operation of Randle's cycle. Feeding, by increasing both insulin and glucose concentration, shifts myocardial metabolism towards preferential carbohydrate usage, both for oxidative energy generation and for glycogen synthesis. During conditions of reduced oxygen supply, the oxidation of all substrates is decreased while anaerobic metabolism is activated. In patients with coronary artery disease and stable angina pectoris, lactate release in the CS can be demonstrated during pacing stress. However, this occurs in only 50% of patients, and no relationship can be demonstrated between lactate production and the severity of ischemia. In patients with chronic angina, a significant release of alanine in the CS and an increased myocardial uptake of glutamate could be demonstrated at rest and following pacing. These two phenomena result from increased transamination of excess pyruvate to alanine with glutamate serving as NH2 donor. In addition, release of citrate (a known inhibitor of glycolysis) in the CS can be demonstrated following pacing in patients with stable angina. The introduction of PET has made it possible to study regional myocardial perfusion and metabolism in humans noninvasively. Two basically different patterns of myocardial glucose utilization have been observed in patients with coronary artery disease studied at rest using 18F-flurodeoxyglucose. In patients with stable angina on exercise but studied at rest, regional myocar- dial glucose utilization was homogeneously low and comparable with that of a group of normals. In contrast, in patients with unstable angina, myocardial glucose utilization at rest was increased even in the absence of symptoms and ECG signs of acute ischemia. In patients with stable angina, a prolonged increase in glucose uptake could be demonstrated in the post-ischemic myocardium in the absence of perfusion abnormalities, and a state of chronic metabolic ischemia is proposed. PET imaging has also allowed prospective differentiation between viable and nonviable segmental function in patients with recent myocardial infarction and in those undergoing coronary artery surgery; in both cases viable segments have relatively maintained glucose uptakes, whereas nonviable segments have depressed glucose uptakes.