Glucose utilization by the isolated rat heart is restricted by flux through the membrane transport, phosphofructokinase and glyceraldehyde-3-P dehydrogenase reactions. These reactions can be accelerated by hormones, such as insulin, by mechanical factors, such as the "garden-hose effect", heart rate, and perhaps ventricular pressure development, and by oxygen deprivation. Glucose utilization is restricted by provision of exogenous non-carbohydrate substrates and by utilization of endogenous substrate stores. Isolated hearts perfused as Langendorff preparations with or without a ventricular drain, working hearts, and ischemic preparations are useful in defining rate-limiting steps and mechanisms of regulation. However, when glucose is supplied as the sole exogenous substrate, its rates of utilization do not reflect the more complex in vivo situation where competing substrates such as fatty acids are preferentially utilized.