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✓ Cerebral oxygen metabolism was studied in the dog at brain temperatures ranging from 37° to 8°C. As brain temperature decreased, the cerebral oxygen metabolism (CMRO2) decreased following the Arrhenius equation. The natural logarithm of the CMRO2 was a linear function of the reciprocal of the absolute (K) brain temperature. Oxygen metabolism, although much decreased, continued at very low brain temperatures. The CSF composition was unchanged after 1 hour at brain temperatures down to 10°C. Circulatory arrest for tolerable periods and longer caused changes only in the CSF potassium concentration. The interval between the onset of circulatory arrest and the beginning of the CSF K concentration increased with decreasing temperature and the rate of CSF K increase was increasingly slower at lower temperatures. At all temperatures the rate of CSF K changed gradually increased with time. The interval before the CSF K started to increase was dependent upon the amount of O2 available in the brain and the length of this interval was inversely proportional to the CMRO2. The amount of CSF K concentration was not clearly related to the tolerable periods of circulatory arrest, but at normal temperatures an obviously increased CSF K following a period of acute cerebral anoxia without CSF hemorrhage may indicate brain damage.