The effect of pentobarbital anesthesia upon the minimal voltage and current required for electrical ventricular defibrillation (the defibrillation threshold) was investigated in dogs. Threshold current, energy, and charge in five dogs averaged 2 per cent, 13 per cent, and 6 per cent less under surgical levels of pentobarbital anesthesia than thresholds in the same animals in the awake, unanesthetized state. In dogs given sufficient pentobarbital to produce apnea and supported by mechanical ventilation, threshold current, energy, and charged averaged 3 per cent, 17 percent, and 2 per cent less than comparable awake values. These differences were far from statistically significant. In a second study, five dogs were kept for 8 to 10 hours at a surgical level of anesthesia with pentobarbital sodium. Defibrillation threshold current, determined at hourly intervals, did not drift outside +/-10 per cent limits. Arterial blood gas measurements revealed a stable, compensated metabolic acidosis in all animals (pH 7.36 +/- 0.06, pCO2 33 +/- 4 mm. Hg, pO2 71 +/- 9 mm. Hg). These data support the validity of defibrillation studies using animals anesthetized with pentobarbital and indicate the stability of the defibrillation threshold under controlled experimental conditions.