Clinical experience with the use of blood transfusions1 in the relief of irreversible insulin coma suggested the possibility that cerebral oxidation—the term is here used to denote oxygen uptake—may at times be depressed or suspended in vivo through the depletion or destruction of certain substances, other than oxygen or lextrose, contained in whole blood. The followng in vitro studies were undertaken therefore to test the effect of certain constituents of the blood on the respiratory activity and survival time of brain tissue and the revival of cerebral oxidation after its spontaneous depression. I it first hoped to imitate the hypoglycemic effect by allowing the tissues to exhaust their own intracellular stores of carbohydrate and then attempting to revive the diminished oxidation at various intervals thereafter by the addition of dextrose, whole blood or serum. In preliminary trials the tissues showed a striking capacity to maintain their oxidation even in