Background: Hemodilution is used widely in clinic as a method of blood conservation. It has been reported that hemodilution may ameliorate ischemia induced brain injury because it increases cerebral blood flow. However, it reduces arterial oxygen content as well. So hemodilution has limited success as a treatment of cerebral ischemia. Objective: To assess the effects of moderate acute normovolemic hemodilution(ANH) on regional cerebral blood flow(rCBF) and cerebral infarct size in a rat model of focal cerebral ischemia. Design: Completely randomized, controlled experiment. Setting and Participants: Beijing Neurosurgical Institute. Sixteen male Wistar rats weighing 300-350 g were used. Interventions: Sixteen Wistar rats were randomly assigned into two groups: control group(n = 8) and ANH group(n = 8). The hematocrit (HCT) of the animals in ANH group was reduced to about 30.7%. The left middle cerebral artery(MCA) was exposed transcranially in both groups. The animals underwent permanent occlusion of the left middle cerebral artery(MCAO) with microbipolar forceps. Cortical rCBF was monitored in both groups before and during the initial 2-hour MCAO with laser Doppler flowmetry (LDF). The infarct size of each brain was determined at 24 hours after MCAO by an experimenter who did not know the grouping. Main Outcome Measures: General physiological data collected in both groups. Changes of rCBF during the initial 2-hour MCAO in ANH group and cerebral infarct size at 24 hours after MCAO were assessed in both groups. Results: In ANH group, hemodilution reduced HCT from 47.28% to 30.74%, and CaO2 from(8.90 ± 0.57) mmol/L to(5.91 ± 0.74) mmol/L(P < 0.01). rCBF reduced to 55.77% of baseline value in the ANH group and 28.5% in the control group during the 2-hour MCAO. There was a significant difference between the two groups(P < 0.01). The total area of infarction was 40.01% in the ANH group and 51.19% in the control group. There was a significant difference between the two groups(P < 0.05). Conclusion: Moderate ANH increases cortical rCBF and decreases brain infarct size in a rat model of focal ischemia. It has a protective effect on cerebral ischemia.