Acute occlusion of a middle cerebral artery (MCA) reduces cerebral blood flow in normotensive Wistar-Kyoto rats (WKY) and in stroke-prone spontaneously hypertensive rats (SHRSP). The goal of this study was to determine whether MCA occlusion produces a sustained reduction in cerebral blood flow or whether collateral vessels restore blood flow to normal levels. We measured blood flow through cerebral collateral vessels to the territory of the occluded MCA and to homologous tissue of the other hemisphere in WKY 1 month after occlusion of the MCA. Cerebral blood flow, measured with microspheres, was restored to normal levels under control conditions in the territory of the occluded MCA. During vasodilatation produced by seizures, blood flow and vascular conductance were increased to similar levels in tissue distal to the site of MCA occlusion and in the homologous tissue of the other hemisphere. MCA occlusion did not produce infarction in any of the WKY. In contrast, 1 month after MCA occlusion in SHRSP, a large atrophic infarct was invariably present in the territory of the occluded MCA. The number of collateral vessels to the territory of the MCA do not differ in SHRSP and WKY. Internal diameter and orientation of the anastomotic vessels differ in SHRSP and WKY. We conclude that, after 1 month of MCA occlusion, changes in the collateral vessels supplying the territory of the occluded MCA in WKY were sufficient to restore blood flow to normal under control conditions and to virtually normal levels during vasodilatation.(ABSTRACT TRUNCATED AT 250 WORDS)