The dog brain can be surgically isolated and will remain viable when supplied with compatible donor blood through the internal carotid artery and a branch of the Internal maxillary artery. Changes in blood bicarbonate, chloride, sodium, potassium, and arterial pressure are reported for isolated brains perfused at a physiologic pH in the presence of a normal or reduced PcO2- Tne concentration of all components tends to increase from 4-5%/hr due to evaporation of water from the system. Both serum sodium and potassium increase in excess of that attributable to evaporation, sodium possibly due to movement of sodium with lactate from the erythrocyte, and potassium due to hemolysis. Low PCO2 causes an increase in cerebrovascular resistance even If the blood is slightly acldotic. The high perfusing hematocrit, formation of platelet aggregates, and release of 5-hydroxy-tryptamine by the platelets may be responsible for the further increases in perfusion pressure observed during the study.