This report covers a summary of 300 animal experiments during profound hypothermia which stresses (1) the acceptable animal survival rate, (2) the favorable metabolic state afforded by this technique and (3) the protective action of profound hypothermia against even severe oxygen deprivation. The technique of profound hypothermia induced and controlled with a pump oxygenator has been shown to be feasible by these studies and has now been widely accepted for clinical application. It has become increasingly evident that, though long periods of circulatory arrest can be achieved, flow of oxygenated blood ought not to be stopped unless the operation demands it. The basis for this statement rests upon the observations that oxygen is utilized during profound hypothermia. There are few cardiac operations where prolonged circulatory arrest is ever needed.