Stroke is the second most common cause of death and ischemic stroke is the largest subtype [1–4]. To investigate the mechanism of cerebral ischemic injury as well as to develop effective therapies to this disease, the use of ischemic stroke animal models is very important. Rodents, especially mice, are widely used for this purpose [5]. Clinically, the occlusion of middle cerebral artery is most of the cases in ischemic stroke [6]. Therefore, middle cerebral artery occlusion (MCAO) is a representative model for ischemic stroke. It is well known that there are many factors exert important effects on the outcome in the MCAO stroke model, including the age and sex of the animal used [6]. Here, we examined the outcomes of MCAO in the different strains of mice. Four different strains of mice (25±2 g in each group) were used in this study. Male ICR mice (n= 33), KM mice (n= 25), BALB/c mice (n= 31), and C57BL/6 mice (n= 29) were purchased from Sino-British SIPPR/BK Lab Animals (Shanghai, China). All animals were used in accordance with the guidelines of Second Military Medical University for Animal Care. The animals were anesthetized with chloral hydrate (300 mg/kg) and remained anesthetized during the following operation. Then, the mice were subjected to MCAO as previously described [7] without reperfusion. After 24 h, neurological deficits were examined using a 5-point scale [7]. Mice were then sacrificed. Brain slices were prepared and stained with 1% 2, 3, 5-triphenyltetrazolium chloride (TTC, Sinopharm Chemical Reagent Co. Ltd. Shanghai, China)[8]. The infarction size was measured using an image analyzer (Image J 1.41; NIH, Bethesda, MD, USA). Data are expressed as mean±SD and analyzed with one-way analysis of variance (ANOVA) followed by Dunnett’s t-test for pairwise comparison. During the following 24 h, 6, 6, 25, and 4 mice died in ICR, KM, BALB/c, and C57BL/6, respectively. The mortality is 18%, 24%, 81%, and 14%, respectively (Table 1). Then, all these brains of dead mice were removed, and signs of hemorrhage, edema or infarction were examined. The data showed that no animals died because of anesthetic or surgical accident. Edema or infarction but