Cerebral infarction was produced in rats by a combination of transient unilateral common carotid artery occlusion and systemic hypoxia. Horseradish peroxidase (HRP) and Evans blue were given 5 minutes prior to sacrifice to assess the integrity of the blood-brain barrier (BBB) at 1 minute, 30 minutes, and 2 hours following the ischemic insult. There was immediate permeability to HRP in the early (1 minute and 30 minutes) post-ischemic period, whereas Evans blue was not seen until the late (1.5 to 2 hours) post-ischemic period. Ultrastructural examination showed two routes of barrier permeability to HRP. In the early post-ischemic period HRP was transported by pinocytosis through endothelial cells in areas of brain containing ischemic neurons. In the late post-ischemic period, HRP diffusely leaked into the brain through the necrotic walls of vessels in areas of infarction. In contrast to previous reports, these results show that the BBB becomes permeable immediately following hypoxia-ischemia. In addition, this study shows that BBB permeability to HRP during cerebral ischemia occurs through two mechanisms: an active, energy-requiring permeability through enhanced pinocytosis within endothelial cells and a passive leakage of protein tracers through necrotic vessel walls.