An attempt is made to evaluate those portions of low molecular hydrophilic solutes that obey apparent passive transfer principles at the blood cerebrospinal fluid (CSF) barrier under steady state conditions. Twenty-four electrolytes, metabolites, and amino acids are determined in serum and CSF obtained simultaneously. The correlation found between the serum/CSF concentration ratios and the molecular radii of most compounds is indicative of the prevalence of passive transfer processes in CSF formation, as has been already demonstrated for proteins. The level of some constituents in the primary filtrate may be secondarily increased by influx from the adjacent extracellular space, e.g. glutamine and creatinine, or decreased by specific uptake from the CSF compartment, e.g. glycine. The CSF levels of all compounds are dependent upon the actual state of the blood CSF barrier as evaluated by the serum/CSF concentration ratios of albumin and alpha 2-macroglobulin. The results do not support the assumption that CSF may be used to test the permeability state of the blood brain barrier propery.