The influence of thyroxine upon n the cardiac uptake of catecholamines was investigated in rabbits. A single injection of thyroxine(1.0m/kg) into rabbits did not affect the concentration of myocardial catecholamines. However, this dose of thyroxine greatly increased the cardiac uptake of catecholamine following injection of 2.0mg of norepinephrine as compared to that of untreated normal animals and it remained elevated for several hours. Similarly thyroxine also enhanced the accumulation of myocardial catecholamines following administration of dopa(60-80mg/kg) and epinephrine(1.0-1.5mg/kg).