31P NMR studies of creatine phosphokinase (CPK) kinetics using saturation transfer techniques are reported. The phosphocreatine (PCr) and ATP levels in perfused hearts was altered experimentally by stopping the flow of perfusate (ischemia) to the heart for 35 min periods, followed by reperfusion to produce stable levels of performance. Utilization of energy by the heart was altered by administration of 25 mM KCl in the perfusate, which arrests contraction of the myocardium. Compared with control heart studies, the unidirectional rates measured during ischemia and KCl arrest are altered. The rates observed in the control experiments indicate that the CPK system is not in a steady state. This apparent deviation from steady-state conditions is ascribed to the existence of intracellular compartmentation of ATP.