The ability of horseradish peroxidase (HRP) to cross the blood-brain barrier (BBB) was studied in dogs by electron microscopy after 18 minutes of complete global brain ischemia (CGBI). The first BBB opening occurred 30 minutes after the recirculation was established ; This was associated with the increase of HRP-containing pinocytotic vesicles (HRP-PV), but there was no endothelial cell damage. There was no BBB opening 3, 6, and 24 hours after the recirculation ; This was determined by absence of increase in HRP-PV and lack of endothelial cell damage. The second BBB opening occurred 48 hours after the recirculation ; This was associated with disruption of the tight junction of the endothelial cell, lack of increased HRP-PV and destruction of the endothelial cell menbrane. These results suggest that the first BBB opening is due to increased pinocytotic transport by reactive hyperemia and the second BBB opening is due to endothelial cell damage by CGBI itself.