Brain cooling was produced in dogs to 21[degree] to 20[degree]C by an extracorporeal circulation utilizing the carotid arteries and jugular veins. At these brain temperatures, cortical electroactivity was abolished and arterial-venous oxygen differences reached zero. Survival was predicated upon perfusion flows from 6 cc/minute/kg in the normothermic animal to 10 cc/minute/kg in the hypothermic preparation. Brain-body circulatory mixing averaged 15 to 30% as measured by radioisotope techniques.