An unanaesthetized cat model of acute focal cerebral ischaemia has been established by the technique of transorbital snare ligature for middle cerebral artery (MCA) occlusion. The model was used to investigate the patterns of changes of regional cerebral blood flow (rCBF) for up to 16 h following MCA occlusion by the hydrogen clearance technique and to explore the correlation among changes of rCBF, neurological deficits, and pathological changes. The animals were divided into 2 groups according to the size of infarct which was identified by 2% triphenyl tetrazolium chloride (TTC) solution stain. Infarct larger than 10% of coronal section surface of ipsilateral hemisphere was found in 18 cats (Group A), and a smaller infarct was found in the remaining 7 cats (Group B). Between these 2 groups, there was a statistically significant difference in the average rCBF value of the ipsilateral MCA territory. The increasing grade of contralateral paralysis correlated well with the decreasing rCBF in the ipsilateral MCA territory, and the increasing grade of cerebral hemispheric swelling was directly proportional to the increasing grade of paralysis. Three patterns of changes of rCBF were observed: persistent severe ischaemia (15 cats, Group A); persistent mild to moderate ischaemia (7 cats, Group B); and immediate severe ischaemia followed by early post-ischaemic hyperaemia and death (3 cats, Group A). During the initial stage of ischaemia, a significant decrease in rCBF of the contralateral hemisphere was observed in both groups, however, in Group A rCBF gradually increased to preocclusion level, while Group B presented a further decrease in rCBF suggesting the occurrence of interhemispheric diaschisis. Thus, Group B appeared to take advantage of diaschisis during the late phase of infarct development, as well as the substantial collateral flow from the surrounding anterior and posterior cerebral artery territories.