This study investigates the fine structural localization of a blood-brain barrier to exogenous horseradish peroxidase in the cerebral cortex of mice. The authors used a recently developed technique to localize peroxidase with the electron microscope after intravenous injection of the enzyme. Brains were fixed by immersion or vascular perfusion 10-60 minutes after administration of various doses of peroxidase. The results showed that exogenous peroxidase was localized in the lumina of blood vessels and in some micropinocytic vesicles within endothelial cells but was not found beyond the vascular endothelium. Micropinocytic vesicles were few in number and did not appear to transport peroxidase, suggesting that tight junctions between endothelial cells prevent its intercellular passage. The findings localize a "barrier" to the passage of peroxidase at the endothelium of vessels in the cerebral cortex, which is distinct from the barrier observed in peripheral capillaries. The significance of these findings is discussed, particularly in relation to similar studies in heart and skeletal muscle. The study highlights the importance of tight junctions and the low frequency of vesicles in the endothelium of cerebral vessels in maintaining the blood-brain barrier.This study investigates the fine structural localization of a blood-brain barrier to exogenous horseradish peroxidase in the cerebral cortex of mice. The authors used a recently developed technique to localize peroxidase with the electron microscope after intravenous injection of the enzyme. Brains were fixed by immersion or vascular perfusion 10-60 minutes after administration of various doses of peroxidase. The results showed that exogenous peroxidase was localized in the lumina of blood vessels and in some micropinocytic vesicles within endothelial cells but was not found beyond the vascular endothelium. Micropinocytic vesicles were few in number and did not appear to transport peroxidase, suggesting that tight junctions between endothelial cells prevent its intercellular passage. The findings localize a "barrier" to the passage of peroxidase at the endothelium of vessels in the cerebral cortex, which is distinct from the barrier observed in peripheral capillaries. The significance of these findings is discussed, particularly in relation to similar studies in heart and skeletal muscle. The study highlights the importance of tight junctions and the low frequency of vesicles in the endothelium of cerebral vessels in maintaining the blood-brain barrier.