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Profound hypothermia (15 C) was induced in 78 dogs anesthetized with either halothane or ether. Various drugs (propranolol, MJ 1999, P-286, alpha-methyl-tyrosine and metaraminol) were given to inhibit or abolish adrenergic hyperactivity. All dogs except those treated with P-286 had increased levels of circulating catecholamines during cooling. Circulating catecholamines appeared to be more important in the production of ventricular fibrillation than tissue stores. Significant protection against ventricular fibrillation was provided by propranolol and P-286 with ether anesthesia, and by MJ 1999 with halothane anesthesia. Elevation of central venous pressure occurred at 18–20 C and was greatest in dogs treated with beta-adrenergic blocking drugs and P-286.