Three series of dog experiments were carried out to study the effect of artificial hypothermia on the cardiac activity and main hemodynamic parameters. In the first series (120 experiments), the dogs were exposed to immersion cooling. In 104 experiments of this series, the whole animal body was cooled to 18-20 degrees C whereas in 16 experiments with the same rate of body hypothermia, selective heart normothermia was maintained by means of an original method of indirect cardiosynchronized warm blood perfusion into the coronary artery. In the second series (6 experiments), the animals were cooled to the same temperature by the combined method: down to 26 degrees C similarly to the first series and then to 18-20 degrees C by means of extracorporeal circulation. In the third series, the animals' head was exposed to an isolated deep hypothermia down to the brain temperature 18-20 degrees C under stable normothermia of the whole body. It has been established that material changes in the phasic structure of the cardiac cycle and in hemodynamic parameters (deterioration of myocardial contractility, decreased cardiac output, etc.) occur only under total body hypothermia, including the hear. A conclusion is made that the changes recorded are consequent on adverse action the low temperature exerts directly on the myocardium and its metabolism.