A model of hyperosmotic opening of the blood-brain barrier(BBB)was successfully made by infusion of 1.4M mannitol into the internal carotid artery of rabbit. We selected lanthanum nitrate as a tracer. Ultrastructural changes of hyperosmotic opening of the BBB were observed through injection of lanthanum nitrate into blood vessels or through impregnation of small blocks of brain tissue in lanthanum solution. Electron microscopy showed that lanthanum nitrate passed through the opening of endothelial tight junction from the blood into the subendothelial layers and the extracellular spaces of the brain, or from the outside into the inside of blood vessels. The data suggest that the morphological basis of osmotic opening of BBB is the reversible widening of the tight junctions between endothelial cells, and that lanthanum nitrate infusion has an advantage over block impregnation.