The effects of erythrocyte function on cerebral ischemia were studied from the perspective of filtration capability (deformability) and total oxygen delivery. Erythrocytes of Wistar rats were divided into three groups: fresh blood, preserved blood, and activated preserved blood (preserved erythrocytes in which total oxygen delivery was increased with phosphoenolpyruvate). Filtration capability was calculated and evaluated by a filtration technique, while total oxygen delivery was evaluated by calculating the efficiency of oxygen delivery from an oxygen dissociation curve. Exchange transfusion was performed in spontaneously hypertensive rats with each of these groups at the same hemoglobin concentration. Bilateral carotid arteries were then ligated for one hour, and, cerebral blood flow and concentrations of adenine nucleotide and lactic acid in cerebral tissue were measured. The filtration capability of both preserved blood and activated preserved blood decreased to about one-third that of fresh blood. The efficiency of oxygen delivery after blood transfusion decreased to 17% with preserved blood, which was about half that (35%) with fresh blood. In the group of with activated preserved blood, this rate increased to 52%. The adenylate energy charge (E.C.) in brain tissue markedly decreased to 0.27 with preserved blood, which was much lower than the value (0.74) with fresh blood. With activated preserved blood, the E.C. value was 0.39. Thus, the decrease in E.C. was mild. With regard to the lactic acid concentration in brain tissue, the value with fresh blood was 12 mumoles/g tissue, while that with preserved blood was 24 mumoles/g tissue, and that with activated preserved blood was 18 mumoles/g tissue. With respect to cerebral blood flow, the values with preserved blood and activated preserved blood decreased to about one-half that with fresh blood. These results suggest that an increase in total oxygen delivery by erythrocytes contributes to improving energy metabolism in brain tissue during cerebral ischemia, and particularly during poor microcirculation.