Study of 19 deaths associated with anesthesia during which the electrocardiogram was monitored revealed that, in at least nine of these patients, ECG changes were not ominous during the initial time in which cardiac output was critically depressed. Dissociation between electrical and contractile cardiac function is the physiological mechanism which probably accounted for the above observation. To demonstrate this, the electrical and contractile actions of the heart were experimentally uncoupled in dogs by simultaneous injection of methoxamine hydrochloride and disodium edetate during monitoring of ECG, electroencephalogram, and femoral-artery and left ventricular pressures. The findings showed that the ECG was usually satisfactory for four to six minutes after cardiac output was absent and marked EEG changes had developed. The clinical and laboratory observations emphasize the need for awareness of the inherent limitations of the ECG as a monitor of total cardiac function.