The concentration ratios of several proteins between serum and cerebrospinal fluid (CSF) can be correlated more easily to hydrodynamic volumes than to molweights. Hydrodynamic radii were calculated from literature diffusion data and determined by analytical column chromatography and gel electrophoresis. Discrepancies between molweight and hydrodynamic volume become evident, especially with the high-molecular proteins because of stronger conformational variability in this size range. The permeability of the blood-CSF-barrier is compared to the blood-lymph-barrier and the results are discussed with reference to current views about capillary permeability. The basement membrane, exposed to the capillary lumen by endothelial fenestrations is considered the most likely site of protein passage at the blood-CSF-barrier.