To define the factors influencing the success of emergency ventricular defibrillation, we identified 52 patients defibrillated at the University of Iowa Hospital during 1974-1976. Thirty-eight patients were successfully defibrillated at least once; 14 could not be defibrillated, despite multiple attempts. Comparisons between these groups revealed no significant differences in body weight, heart weight, energy per kilogram of body weight and energy per gram of heart weight. Factors that militated against successful defibrillation included a prolonged delay before the first defibrillatory shock (successful 7 + 7 minutes (SD); unsuccessful 17 ± 13 minutes, p < 0.001), acidosis (successful pH 7.36 ± 0.22; unsuccessful pH 7.23 ± 0.12, p = 0.05) and hypoxia (successful Po2 100 + 98 torr; unsuccessful Po, 40 ± 67 torr; p = 0.06). These three conditions tended to occur together in individual patients. Metabolic factors are important in determining defibrillation success; however, the role of high-energy doses is uncertain.