The effect of magnesium chloride on the ventricular fibrillation threshold (VFT) and the threshold for the ventricular premature contraction (VPCT) was studied in 20 dogs. Seven of the dogs were pre-treated with digitalis and four were in the form of heart-lung preparations. In the anesthetized, intact dogs, the VPCT was 0.19 +/- 0.01 mV. After treatment with magnesium chloride (100 mg. per kilogram intravenously), the VPCT increased by 53% (P less than 0.01). In the same group of animals, the VFT averaged 0.50 +/- 0.06 mV., which more than doubled after administration of magnesium. The threshold of VPC in the digitalis-treated dogs measured 0.18 +/- 0.01 mV; this value doubled after magnesium. The VFT in the digitalized dogs also increased after magnesium; however, resistance to electrical defibrillation was encountered in this group. In the heart-lung preparations, VPCT improved by 72% and a gain of 131% in the VFT followed magnesium administration. The results suggest that magnesium increases the ventricular threshold of arrhythmias in normal, denervated (heart-lung preparations) and also digitalis-treated hearts and, thus, indicate its usefulness in the treatment of ventricular arrhythmias.