An artificial oxygen carrier, liposome-encapsulated hemoglobin (LEH), protective in a rodent stroke model, was quantitatively evaluated in monkeys. Serial positron emission tomography studies using the steady-state 15 O-gas inhalation method were performed to quantify O 2 metabolism, which was compared based on the infarction extent and immunohistochemical evaluation in 19 monkeys undergoing middle cerebral artery occlusion (3 h), infusion of various LEH doses (n ϭ 11), empty liposome (n ϭ 4), or saline (n ϭ 4) 5 min after the onset of ischemia, and reperfusion for 5 h. There was no significant difference in O 2 metabolism until 3 h after reperfusion, when the cerebral metabolic rate of O 2 (CMRO 2 ) was significantly less suppressed in the cortex [mild suppression in CMRO 2 (71-100%) of preischemic ipsilateral control as in the ischemic penumbra: 64.7 Ϯ 14.3% in empty liposome versus 32.4 Ϯ 7.9% in LEH (2 ml/kg) treatment, P Ͻ 0.05] but not in basal ganglia. Immunohistochemical studies showed a reciprocal expression of microtubular-associated protein II expression in the cortex and LEH deposition in basal ganglia, suggesting the LEH perfusion, but not deposition, afforded the protection. Doseresponse studies revealed that as little as 0.4 ml/kg LEH (24 mg/kg hemoglobin) was effective in preserving CMRO 2 , whereas 2 and 10 ml/kg were protective in significantly reducing the area of infarction as well, by 66 and 56%, respectively, compared with animals receiving saline. CMRO 2 and histological integrity were better preserved early after 3-h occlusion and reperfusion of the middle cerebral artery of monkeys receiving LEH early after onset of ischemia.Although ischemic stroke is a major cause of morbidity and mortality in developed societies (Broderick et al., 1998), medical attention is often delayed because of low public awareness of stroke symptoms (Williams et al., 1997). Increased risk of hemorrhagic transformation (Lapchak, 2002) limits the therapeutic time window and makes it more difficult to apply reperfusion therapy (NINDS rt-PA Stroke Study Group, 1995), as with coronary circulation. Although cell-free hemoglobin has been proposed to reduce ischemic injury, the associated toxicities have limited its utility (Natanson et al., 2008). Liposome-encapsulated human hemoglobin (LEH) (Ogata Y, 2000) has been developed as an artificial O 2 carrier that is functionally and structurally similar to red blood cells (RBCs). The lipid bilayer of encapsulated hemoglobin prevents renal clearance, extravasation, and direct contact of hemoglobin with vascular smooth muscle (Ogata, 2001), thus prohibiting vascular constriction, prolonging the in vivo halflife of LEH (Kaneda et al., 2009), and reducing the dosage required for beneficial effects (Kawaguchi et al., 2007). Although liposomes are much smaller (230 nm), their O 2 -carrying capacity is equivalent to human RBCs under room air respiration (Kaneda et al., 2009). These physical characteristics prompted us to examine the therapeutic potential of LEH, not as an...