Considering the possibility of a paracellular pathway for edema resoltuion, we studied the intracerebral movement of proteins and ionic lanthanum in rats with experimental hydrocephalus. Hydrocephalus was induced by injection of kaolin suspension into the cisterna magna. After induction of hydrocephalus, horseradish peroxidase (HRP), microperoxidase (MP), or lanthanum chloride (LaCl3) were perfused into the ventricle system. HRP and MP were localized mainly in the intercellular spaces between ependymal cells, glial cells, and in perivascular spaces and were restricted by endothelial tight junctions. Ionic lanthanum (La3+), however, penetrated these tight junctions and moved between the blood and CSF cavities by paracellular pathways. These findings indicate that in obstructive hydrocephalus, the tight junctions may constitute part of a paracellular pathway for the transendothelial movement of small solutes, although they prevent the movement of larger molecules.