The heart rate of perfused hearts of 20 thyroxin-ized rabbits and the heart-lung preparation of thyroxin-ized dogs (No. not stated) was considerably above normal. Similarly, when the heart of a small dog was transplanted into the neck of a large dog by means of anastomosis of blood vessels, thyroxin evoked definite tachycardia of the transplanted heart. Apparently the tachycardia of experimental hyperthyroidism is not dependent on the central nervous system, but on a peripheral mechanism.