The respective influences of ischemic neuronal damage and infarction on the development of abnormal blood-brain barrier (BBB) permeability and cerebral edema were evaluated in a rat model of temporary four-vessel occlusion in which ischemic neuronal damage with only infrequent infarction is produced. Survival times ranged from 40 minutes to 5 days after ischemia. Evans blue and horseradish peroxidase (HRP) were given before sacrifice. The majority of brain showed moderate ischemic neuronal damage inthe striatum. In these areas there was neither leakage of Evans blue nor extravasation of HRP. Astrocytic processes were moderately swollen. Large, grossly-visible unilateral infarcts were present in only 5 animals, and all showed abnormal BBB permeability of HRP which occurred via enchanced pinocytosis, and occasionally via diffuse leakage through necrotic vessels. Astrocytic processes were markedly swollen and their plasma membranes were disrupted. Whole brain and regional water content in a parallel series of animals were measured from 15 minutes (min) to 48 hours (h) postischemia. They showed a transient, 1% increase in whole brain water content from 15 to 60 min postischemia, but no increase in regional water content at any postischemic interval. These studies suggest that ischemia produces BBB permeability to large molecules, and sustained cerebral edema only when the process damages blood vessels and astrocytes; neuronal necrosis alone is insufficient.