The safe limit of normothermic arrest of the canine heart, after which marked deterioration in function occurs, is investigated in the intact animal, using restricted bypass support. The results show a distinct difference between two groups of dogs with 30 and 60 min warm ischaemia respectively, which is primarily reflected in the ability to sustain the circulation, and also in the electrocardiogram, left ventricular end-diastolic pressure, and the rate of oxygen consumption.