The correlation between the changes of regional cerebral blood flow (rCBF) and the histological changes were examined using the middle cerebral arterial (MCA) occlusion model in cats. A total of 24 adult cats were tracheostomized and anesthetized by inhalation of halothane. The right MCA was clipped by the transorbital approach. Two hours after the application, the clip was removed and the brain was recirculated for two hours. Then, the brain was perfusion-fixated and the histological studies were carried out. The animals were separated into two groups according to the severity of histological damages using light and electron microscope. Severe cortical damage was present in 8 cats (Group A). In the remaining 16 cats, little or no cortical damage was found (Group B). The averaged rCBF values before occlusion were 45.4 +/- 2.3 ml/100 gm/min in group A and 46.5 +/- 1.6 in group B, showing no statistically significant difference. Between these two groups, however, there was a statistically significant difference in the averaged rCBF values during the ischemic period. In group A, the averaged rCBF values during MCA occlusion was only 6.8 +/- 0.9 and in group B, it was 25.3 +/- 0.8. In the recirculation period, there was a prompt and uniform recovery of rCBF in group B. Whereas in group A, a marked diversity of rCBF ranging from oligemia to hyperemia ensued. This is presumably a reflection of inhomogeneous blood flow, or patchy non-filling of the cerebral cortex. The critical values of rCBF as to the occurrence of severe cortical damage in two-hours MCA occlusion is considered to lie between the lowest value of group B and the highest value of group A, i.e., around 12--15 ml/100 gm/min.