Objective: Our purpose was to assess the cerebral blood flow distribution and resulting grade of hypoxic-ischemic brain damage in newborn rats. Methods: Seven-day-old Wistar rats (n = 75) underwent left common carotid artery ligation followed by 2 hours hypoxia (8% oxygen in nitrogen) at 33degreesC. The control animals were exposed to hypoxia without ligation (n = 8). Colored microspheres of 15 mum in diameter were administered into the left cardiac ventricle percutaneously at the end of hypoxia. They were killed 24 hours after induced injury. Brain sections 2 mm in thickness were removed for microtubule-associated protein 2 (MAP-2) staining, and remaining parts were separated into left and right hemispheres for counting the microspheres. The blood flow distribution to the ligated side was expressed as the difference from the non-ligated control side. Severity of MAP-2 disappearance was ranked as normal, mild or severe. Results: In the control rats, there was no loss of MAP-2 staining. The blood flow equally distributed into both cerebral hemispheres. The cerebral blood flow distribution on the side of carotid artery ligation decreased by 44.7 +/- 21.9% in the mildly damaged group and 65.8 +/- 16.8% in the severely damaged group. Conclusion: The greater the percentage difference of blood flow distribution from the non-ligated side, the more severe the brain damage.