The control of energy metabolism within the intact heart is being investigated using a variety of techniques and preparations. The purpose of these studies is to determine the mechanism involved in the interaction of cellular energy conversion, by oxidative phosphorylation or glycolysis, with cardiac muscle contraction and subsequent pumping of blood. These studies are being conducted using a combination of classical techniques as well as state of the are non-invasive methods involving nuclear magnetic resonance (NMR) and optical spectroscopy to monitor the intracellular metabolic effects of changes in cardiac work output in vivo. NMR is used to monitor the cellular concentrations of adenosine di- and tri- phosphates, inorganic phosphate, creatine phosphate and pH. The mitochondrial redox state and oxygenation, in vivo, is being monitored using opitcal spectroscopy. These experiments have demonstrated that the phosphate metabolites do not change during physiological alterations of steady state work output. Rapid transient studies (resolution <8 sec) also demonstrated that no transient change occurred in these metabolites during a work jump. Changes in the high energy phosphates occurred only when the work performed exceeded the metabolic capacity of the tissue. These data suggest that these phosphate metabolites are to the key cytosolic intermediates in the regulation of myocardial oxidative metabolism as once believed. Opitcal data suggest that the mitochondrial NADH redox state does change appropriately (i.e. becomes more reduced) with an increase in work. These data suggest that the control energy conversion in the heart is maybe controlled at the level of substrate oxidation or oxygen delivery to the myocardium.