Sudden occlusion of the middle cerebral artery (MCA) in normotensive rats increases blood flow through anastomosing branches into the territory of the occluded artery. Three weeks after MCA occlusion, anastomoses to anterior cerebral branches are increased by more than 50% in luminal diameter. One month after MCA occlusion, blood flow and blood flow reserve to the territory of the occluded MCA are returned to normal levels. In stroke-prone spontaneously hypertensive rats (SHRSP), the anastomoses are significantly narrower and blood flow through the anastomoses is less than in normotensive rats. Tissue infarction invariably develops in the territory of the occluded MCA in SHRSP. We propose that the luminal width of the anastomosis is a major determinant of blood flow into the territory of the occluded artery and of the amount of tissue protected from infarction by collateral circulation.