A phosphorus 31-nuclear magnetic resonance method was used to study the effect of exogenous phosphocreatine on the isolated perfused rat heart. The hearts were chemically arrested by St. Thomas' Hospital solution and made totally ischemic for 35 minutes at 37 degrees C. In the presence of phosphocreatine, 10 mmol/L, during ischemia, almost complete recovery of heart function and phosphocreatine content and 61% recovery of adenosine triphosphate content were observed after 30 minutes of postischemic reperfusion; in the control experiments without phosphocreatine, contractile function, intracellular phosphocreatine, and adenosine triphosphate contents were restored to 33%, 43%, and 26% of their normal values, respectively. Ultrastructural studies with a lanthanum tracer method showed remarkable protection of sarcolemma against ischemic injury by exogenous phosphocreatine at the level of the glycocalyx.