Mongrel dogs cooled to terminus exhibited a 91% incidence of ventricular fibrillation. Animals receiving 25% glucose or 50% sucrose intravenously during cooling did not fibrillate but terminated in asystole. In untreated cooled animals, plasma concentrations decreased, and plasma calcium and sodium were unaltered. Dogs receiving hypertonic glucose or sucrose had decreased plasma calcium and sodium levels, but normal potassium concentrations at lower body temperatures. The possible mechanisms by which hypertonic fluids decrease excessive cardiac excitability are discussed.