Goldfish CNS was fractionated by differential and density gradient centrifugation. The fractions obtained were characterized by marker enzymes typical of various subcellular organelles. They were further analyzed by radioimmunoassay for their contents of ependymins, two CNS glycoproteins known to participate in biochemical reactions after learning events. Ependymins were shown to be major constituents of the soluble cytoplasm (5.6% of the total protein content). The nuclear fraction was virtually devoid of ependymins (0.6% of protein). Small amounts were observed in the crude synaptosomal and microsomal fractions (1.0 and 3.5%, respectively). The highest steady-state concentration of ependymins, however, was measured in the brain extracellular fluid (15.6% of the protein), including the CSF. The specificity of the distribution was examined by intracerebroventricular injection of 125I-labeled ependymins as exogenous marker substances. No indication of an artificial redistribution of the radiolabel during homogenization and fractionation was obtained. The exogenous analogues of ependymins were, however, incorporated in vivo into organelles recovered in the nuclear and crude synaptosomal fractions. Our results suggest that ependymins may interact with synaptic membranes from the extracellular fluid, although so far no evidence for a specific receptor-type binding site could be obtained in vitro.