The effect of severe hyperthermia on the circulatory function was studied in dogs. Arterial pressure was maintained at the normal level, cardiac output increased at the core temperature of about 40 degrees C. An abrupt fall of the arterial pressure and cardiac output was observed at the rectal temperature of about 41 degrees C. The results suggest that a decrease in the cardiac output during severe hyperthermia is due to the fall of the central venous pressure and to the increase of the vascular compliance.