More than half the deaths due to cardiac disease occur suddenly, with ventricular fibrillation (VF) being the most common presenting rhythm. Current treatment protocols call for immediate countershock of the patient found in VF. While it is clear that immediate electrical countershock is the most effective initial treatment for VF, its effectiveness declines rapidly over time. The present study is to determine the effect of repeated electrical countershocks on the myocardial high energy phosphates. Mixed breed domestic swine are prepaerd with 3-lead ECG to monitor cardiac rhythm. An NMR surface coil tuned to 31P is sutured to the apex of the myocardium. The animal is then placed in the 4.7-Tesla 40-cm magnet and baseline 31P-NMR spectra and ECG tracing recorded. VF is then induced with a 3sec, 60Hz, 100mA thoracic shock, and recording of NMR and ECG continued for 10 minutes. This model simulates a prolonged out-of-hospital cardiac arrest. Current clinical treatment guidelines require 3 consecutive countershocks of increasing energy doses to be administered prior to any other intervention. We use 2 J/kg, 3 J/kg, and 4 J/kg to approximate the human adult energy requirements. EKG and NMR are recorded at 1 minute intervals from the time of the initial countershock until 4 minutes after the final countershock. This project is in its early stages of development, and five animals have so far been studied. These initial pilot studies have allowed us to overcome a number of technical difficulties. It is anticipated that the refinement of the experiments will be completed in the next two or three experiments, so that the entire protocol carried out be continued soon.