It has become increasingly clear that a stroke lesion usually consists of a densely ischemic focus and of perifocal areas with better upheld flow rates. At least in rats and cats, some of these perifocal ("penumbral") areas subsequently become recruited in the infarction process. The mechanisms may involve an aberrant cellular calcium metabolism and enhanced production of free radicals. In general, though, the metabolic perturbation in the penumbra requires better characterization. The objective of this article was to define flow distribution in a rat model of reversible middle cerebral artery (MCA) occlusion, so as to allow delineation of the metabolic aberrations responsible for the subsequent infarction. We modified the intraluminal filament occlusion model recently developed by Koizumi et al. (1986), and described in more detail by Nagasawa and Kogure (1989), adopting it for use in both spontaneously breathing and artificially ventilated rats. Successful occlusion of the MCA (achieved in about 9/10 rats) was judged by unilateral EEG depression in ventilated rats, and neurological deficits, such as circling, in spontaneously breathing ones. CBF in the ipsilateral hemisphere was reduced to nearly constant values after 20, 60, and 120 min of occlusion, flow rates in the focus being about 10% and in the perifocal ipsilateral areas about 15-20% of control (contralateral side). When the filament was left in place (permanent occlusion) 2,3,5-triphenyl tetrazolium chloride (TTC) staining and histopathology after 24 h showed a massive infarct on the occluded side, extending from caudoputamen and overlaying cortex to the occipital striate cortex. Animals recirculated after 60 min of MCA occlusion, and allowed to survive 7 days for histopathology, showed infarction of the caudoputamen (lateral part or whole nucleus) in 5/6 animals and selective neuronal necrosis in one animal. The neocortex showed either infarcts, selective neuronal necrosis, or no damage. There was some overlap between neocortical areas which were infarcted and those which were salvaged by reperfusion. In general, though, both the CBF data and the recovery studies with a histopathological endpoint define large parts of the neocortex as perifocal (penumbral) areas which lend themselves to studies of metabolic events leading to infarction.