Recently we observed inter- and intrastrain differences in cortical infarct volumes after middle cerebral artery (MCA) occlusion. Variations in the anastomoses providing collateral blood supply could account for different lesion sizes. Our objectives were to compare number and internal diameters of the MCA-anterior cerebral artery (MCA-ACA) anastomoses and to determine if the lesion extended beyond branches of the MCA territory into the field of the ACA in the rat strains/lines. Sprague-Dawley rats and Wistar rats from Simonsen Laboratories (SLSD and SLWIS) and Sprague-Dawley rats from Taconic Laboratories (TLSD) and Charles River Laboratories (CRSD) were anesthetized and injected with papaverine and Vultex (white latex) for arterial visualization. Some rats were also subjected to MCA occlusion. Significantly fewer anastomoses were present in SLSD and SLWIS than in CRSD and TLSD (p < 0.05). The mean internal diameters of the anastomoses were not significantly different between the strains/lines (p < 0.05). After MCA occlusion, significantly more (p < 0.05) TLSD and CRSD than SLSD had lesions extending from the MCA field beneath the anastomoses and into the region supplied by the ACA. Neither the number, luminal diameter, nor density of MCA-ACA anastomoses appears to be the limiting factor that differentiates lesion size following MCA occlusion in these particular rat strains/lines. Therefore, factors other than anatomical variations probably account for different lesion sizes.