ONE OF the constant findings in a liverless dog is a progressive tachycardia. Consequently, 24 hours after hepatectomy the heart rate of the animal may be about 250 beats a minute. The animal otherwise may appear normal and walk about the room with little evidence to show that it has been subjected to a major operation. In an effort to elucidate the nature of this tachycardia my colleagues and I decided to connect the circulation of a liverless dog with that of a transplanted heart. If the transplant also shows tachycardia, one can be certain that this is a result of a humoral agency consequent upon removal of the liver. Because the technique of homotransplanting a heart turned out to be feasible, once we became familiar with it, we transplanted a series of 30 hearts, of which 23 survived. They beat on an average for 129 hours, and two hearts