The minute cardiac output was determined on normal trained dogs and narcotized dogs by the Fick principle. 0 consumption was recorded by the Roth modification of the Benedict spirometer, using the Blalock dog mask. Simultaneous right and left heart punctures were used to obtain blood samples. Anoxemia was produced by having dogs rebreathe room air from the spirometer, the CO2 being absorbed by the soda lime. The time of rebreathing varied, according to the size of the dog, from 7 to 45 min. After control periods, O saturation values produced by this method were 70-3%. Anoxemia of severe degree (arterial saturation of 70% or less) produced: (1) slight increase in pulse rate; (2) increased minute ventilation; (3) diminished O consumption (when arterial saturation was less that 50%); (4) uncompensated acid deficit (diminished CO2 content and decreased H-ion concentration); (5) increase of 5-500% in the minute cardiac output. The increase in cardiac output was accomplished by an augmented stroke volume, the mechanism of action being deduced as due to the effect of decreased O tension on the heart muscle.