The present study was carried out to investigate the permeability of normal spinal nerve root sheaths around dorsal and ventral roots in the rat. In vivo studies were performed using Evans blue-albumin and lanthanum chloride as tracers. The Evans blue-albumin complex is macromolecular in size and lanthanum ions are small and easily visible in the electron microscope. Both tracers were injected into the subarachnoid space and 15 min later samples were taken and further processed for detection of tracer. Postmortem studies with lanthanum was also performed. Following fixation by cardiac perfusion with fixative without tracer, lanthanum chloride was added to the fixative and applied directly to exposed spinal cord including the spinal nerve roots. Macroscopical examination showed Evans blue staining of the superficial blood vessels of the spinal cord, but no staining of the parenchyma of either spinal cord or nerve roots. Fluorescence microscopy revealed, in addition to a bright red fluorescence of root sheaths, a faint longitudinally orientated red fluorescence in the endoneurium of the nerve roots, indicating the presence of the dye-albumin complex. In both in vivo and post-mortem lanthanum studies, the tracer was detected between cell layers of the nerve root sheath and in invaginations of the plasma membrane of these cells, as well as inside the nerve root parenchyma. Some of the cells of the sheaths in post-mortem animals were diffusely marked with intracellular tracer. The endo-radicular lanthanum was most often seen superficially, but lanthanum could occasionally be detected deeper in the parenchyma in the post mortem studies.(ABSTRACT TRUNCATED AT 250 WORDS)