Immortalized porcine brain microvessel endothelial cells (PBMEC/C1-2) were used to develop a model for measurement of blood-brain barrier permeation of central nervous system active drugs. Previous studies showed that a system using C6 astrocyte glioma conditioned medium leads to cell layers with transendothelial electrical resistance values up to 300 Ω cm 2 and a permeability coefficient P e of 3.24 +/- 0.14 x 10 - 4 cm/min for U-[ 1 4 C]sucrose, which is in good agreement to published values and thus indicates the formation of tight junctions in vitro. However, commercially available inserts for the Transwell ( R) system were not permeable for highly lipophilic compounds, such as diazepam. Systematic studies with different insert showed, that inserts with a pore width of 1 μm proved to be optimal for permeation studies of lipophilic compounds. Permeability studies with a set of three benzodiazepines further supported this finding.