A model was set up in order to evaluate the importance of hemispheric perfusion pressure when the middle cerebral artery (MCA) is occluded in anaesthetized rats. In 6 animals the internal carotid artery (ICA) was occluded prior to ipsilateral MCA occlusion; in 17 animals the MCA only was occluded; 6 animals underwent the same preparation, but the vessels were left unoccluded. Four days after surgery the infarct volume was measured with a computerized image analyser. The infarcted areas were significantly larger in the ICA + MCA occluded group compared with the MCA occluded group (p less than 0.005), which in turn had larger infarcts than the sham-operated animals (p less than 0.001). These results indicate that patients with hypoperfusion, due to severe ICA stenosis and impaired collateral blood supply, are at higher risk of developing major stroke, when embolism into a cerebral artery occurs, as compared to patients with no, or only minor, reduction in hemispheric perfusion pressure.