Abstract - Polyethylene glycol-conjugated hemoglobin (PEG-Hb) has been proposed as a blood substitute for transfusion due to their plasma expansion and oxygen transport capabilities. The protective effect of PEG-Hb on cerebral hypoxic-ischemic injury was investigated in neonatal hypoxia model and adult rat focal cerebral ischemia model. As intravenously administered 30 min before the onset of hypoxia, PEG-Hb markedly protected cerebral hypoxic injury in a neonatal rat hypoxia model. A similar treatment of PEG-Hb largely reduced the ischemic injury ensuing after 2-h middle cerebral artery occlusion followed by 22-h reperfusion. Consistently, neurological disorder was significantly improved by PEG-Hb. The results indicate that the pharmacological blockade of cerebral ischemic injury by using PEG-Hb may provide a useful strategy for the treatment of cerebral stroke. Keywords: Polyethylene glycol-conjugated hemoglobin, Hypoxia, Focal cerebral ischemia INTRODUCTION The brain is more vulnerable to ischemic insults than any other organs. The level of oxygen in brain depends on cerebral blood flow and arterial oxygen content. Normal cerebral blood flow is approximately 50 ml/100 g/min. When cerebral blood flow drops below a critical level of 10 ml/100 g/min, irreversible brain damage occurs, with its ensuing metabolic derangement (Pulsinelli, 1992). Even when cerebral blood flow falls to 25 ml/100 g/min, neuronal cells become electrically silent, although they remain po-tentially viable (Hossman, 1994). Therefore, augmentation of oxygen delivery into hypoxic/ischemic lesions would be beneficial to prevent further neurological deterioration in stroke. Development of artificial blood substitutes has been mo-tivated by insufficient blood supply, dangers of blood in-compatibility reactions, and/or infectious diseases in ho-mologous blood transfusion. Furthermore, hemoglobin- based oxygen carriers can be sterilized and stored for pro-longed periods of time. For decades, free hemoglobin de-rived from hemolyzed blood or hemoglobin-based oxygen carriers have been designed as temporary substitutes to red blood cells (Winslow, 2008). Artificial oxygen carriers are grouped into hamoglobin-based oxygen carriers and perfluorocarbon emulsions (Spahn and Kocian, 2005). Several of hemoglobin-based oxygen carriers, including cross-linked hemoglobin, polyethylene glycol (PEG)-con-jugated hemoglobin (PEG-Hb), and recombinant hemoglo-bin, have been demonstrated to maintain circulating vol-ume (Hu