Profound hypothermia (15 ℃) was induced in 36 mogrel dogs anesthetized with nitrous oxide-oxygen-d-tubocurarine. Prcpranolol (0.3 mg/kg) or MJ 1999(0.8 mg/kg) was administered intravenously 10 minutes prior to the start of cooling. Significant protection against ventricular fibrillation was provided by MJ 1999, but not by propranolol. With available evidence that hypothermia causes increased catecholamine release and that MJ 1999 is more specific in its beta-adrenergic blocking prcperties than propranolol, the author's findings suggest that ventricular fibrillation during hypothermia is due to beta-adrenergic hyperactivity.