In both accidental and surgical hypothermia, the chief hazard to life is the occurrence of ventricular fibrillation. Glycine, an amino acid,, has been used to inhibit cyclopropane-epinephrine induced arrhythmias, and this compound was evaluated in the prevention of arrhythmias due to lowered body temperatures. Mongrel dogs were used; pH, blood pressure, electrocardiograms, plasma potassium, and blood sugar were determined at frequent intervals during cooling. Control animals, receiving no medication were cooled to terminus and the incidence of ventricular fibrillation was 96%. A second group receiving glycine, 1.5 to 2.0 gm./kg., was cooled to terminus, and the incidence of fibrillation was reduced to 0%. In animals cooled to a reduced temperature of 24° C and subjected to a right thoracotomy, right ventriculotomy, and 10 minutes' venous occlusion, the incidence of ventricular fibrillation was 93%; the frequency of fibrillation in glycine treated animals during hypothermic cardiac surgery was 29%. Glycine treated animals exhibited significantly higher blood sugar and plasma potassium levels during hypothermia. The most likely explanation of the antiarrhythmic effect of glycine in hypothermia appears to be its ability to increase plasma potassium concentration.