Department of Biochemistry and Gray Freshwater Biological Institute, University of Minnesota, Navarre, Minnesota 55392, Department of Surgery, University of Minnesota, Minneapolis, Minnesota 55455, and Department of Medicine, Cardiology Division, Minneapolis VA Medical Center and University of Minnesota, Minneapolis, Minnesota 55455 Received April 3, 1987; Revised Manuscript Received June 30, 1987 abstract: The origin of the nuclear magnetic resonance(NMR)-measurable ATP<= t Pj exchange and whether it can be used to determine net oxidative ATP synthesis rates in the intact myocardium were examined by detailed measurements of ATP «= s P; exchange rates in both directions as a function of the myocardial oxygen consumption rate (MV02) in (1) glucose-perfused, isovolumic rat hearts with normal glycolytic activity and (2) pyruvate-perfused hearts where glycolytic activity was reduced or eliminated either by depletion of their endogenous glycogen or by use of the inhibitor iodoacetate. In glucose-perfused hearts, the Pj-* ATP rate measured by the conventional two-site saturation transfer (CST) technique remainedconstant while MV02 was increased approximately 2-fold. When the glycolytic activity was reduced, the P;—>•