Summary Inhalation of 100% oxygen has been shown to result in the elevation of the T wave in the electrocardiogram in 20 of 26 cardiac cases, and in 2 of a group of 8 normal and miscellaneous cases. When 97 or 96% oxygen with 3 or 4% carbon dioxide was tested, similar electrocardiographic changes were observed although the degree of change was frequently less marked; thus, in 7 of 20 cardiac cases the T wave was distinctly more elevated after inhalation of 100% oxygen than after the oxygen-carbon dioxide mixture. The significance of these observations is discussed. It seems likely that inhalation of 100% oxygen shortens the recovery period of cardiac muscle.