The 72 hour chick embryo heart reacts to progressive hypoxia by the appearance of arrhythmias and asystole. In the presence of 4 per cent CO2, complete anoxia is tolerated for a period of 30 min. with carbon dioxide apparently augmenting the buffering capacity of the plasma clot which supports the heart. A gaseous environment of 4 per cent O2/4 per cent CO2/92 per cent N2induced a steady and entirely reversible state of cardiac stress.