In a previous communication,1 it has been shown that the cardiac output of the dog's heart is increased during an ether anesthetic. This change was associated with an increase in the hydrogen ion concentration of the blood. Boothby2 first suggested that the H-ion concentration regulated the cardiac output of the heart. Douglas and Haldane3 reached the same conclusion from studies on man. A similar view has been expressed by Means.4 Harrison, Wilson and Blalock5 found that the production of respiratory obstruction and the injection of acid were associated with an increase in the H-ion concentration of the blood and an increase in the cardiac output, and that the injection of alkali resulted in the reversal of these two functions. They concluded that the changes in the cardiac output of the heart were dependent on alterations in the H-ion concentration of the blood. The object of