The outpouching of CNS tissues into the entering spinal nerve roots was documented by light and electron microscopy of human and bovine tissues. Astrocytic processes containing large bundles of glial filaments were very prominent in the nerve entry zone and extended for short distances into the adjacent endoneurium of the spinal nerve roots. Antiserum raised to glial acidic fibrillary (GFA) protein stained these glial elements, thereby characterizing the dome-shaped evaginations of CNS tissues into the nerve root entry zones. Antisera to CNS basic protein showed enhanced staining in the nerve entry zone. Analyses of nerve proteins by SDS gel electrophoresis disclosed a prominent 49,000 MW protein in the bovine and human nerve root entry zone. This protein was also prominent in spinal cord white matter, but was not seen in nerve roots which were not admixed with glial tissues. This finding supported the view that a 49,000 MW protein is a glial filaments but is not a component of bovine or human neurofilaments.