<正> A model of hyperosmotic opening of the blood-brain barrier（BBB）was successfully made by infusion of 1.4M mannitol into the internal carotidartery of rabbit.We selected lanthanum nitrate as a tracer.Ultrastructuralchanges of hyperosmotie opening of the BBB were observed through injection oflanthanum nitrate into blood vessels or through impregnation of small blocks ofbrain tissue in lanthanum solution.Electron microscopy showed that lanthanumnitrate passed through the opening of endothelial tight junction from the bloodinto the subendothelial layers and the extracellular spaces of the brain

or fromthe outside into the inside of blood vessels.The data suggest that themorphological basis of osmotic opening of BBB is the reversible widening of thetight junctions between endothelial cells

and that lanthanum nitrate infusion hasan advantage over block impregnation.