Since there is evidence that it is often difficult or impossible to defibrillate vary large patients with the energy delivered by most presently available commercial defibrillators, a research defibrillator designed around two 18,000 joule capacitor banks has been constructed for the study of the defibrillatory energy requirements of large experimental subjects. The defibrillator is being used in the systematic experimental study of the relationship between the various parameters of waveform and the effectiveness of the shock in reversing ventricular fibrillation in 90-110 kilogram calves. A study involving rectangular waveforms is almost complete and we plan to soon initiate a study involving untruncated and truncated exponential discharges. In this study, the waveform variables will be initial current, final current at time of truncation, and circuit time constant. BIBLIOGRAPHIC REFERENCES: Schuder, J.C. and Gold, J.H. Calculator-aided programming of an ultrahigh energy research defibrillator. ASAIO Abstracts, 5:72, 1976. Schuder, J.C. and Gold, J.H. Design of an ultrahigh-energy hydrogen thyratron/SCR research defibrillator. Medical Instrumentation, 10:146, 1976.