Anoxemia in the normal man can safely be carried to the stage of unconsciousness when produced by the process of continuous and gradual reduction of the oxygen in the air breathed. The responses of the body to the gradual oxygen reduction are compensatory to a certain limit. The crisis appears promptly when the compensating limit is reached for any particular person and set of experimental conditions until in rapid succession the following states are passed: (1) Decrease and loss of attention; (2) loss of voluntary control of muscles, and (3) complete unconsciousness but with a degree of reflex control of voluntary muscles. The heart beat and respiratory movements persist through these three cycles. The degree of circulatory efficiency at the onset of unconsciousness is adequate for the moment, but is assumed, in general, to be failing rapidly. We have applied the methods of the electrocardiograph to the study of the