Claudins are components of the tight junctional complex in epithelial and endothelial cells. We characterized the composition of tight junctions in the choroid plexus of the lateral ventricle in the rat brain and tested whether protein kinase C induced changes in their composition. Claudin-1, -2 and -5 were present in the epithelial cells at and near the tight junctions, respectively. In the endothelial cells, claudin-5 was stronger expressed than claudin-1 and -2. Twenty-four hours after the phorbolester injection into the ventricle, claudin-1 immunoreactivity of the epithelial cells was increased and spread to the cytoplasm. The claudin-2 and -5 immunoreactivities were reduced. These findings are consistent with an influence of protein kinase C on the composition of the tight junctions in the choroid plexus.