The localization of hyaluronectin was determined by immunofluorescence and immunoperoxidase methods, in the rat, the sheep and the human. The study of the peripheral nervous system revealed the localization of this protein at the node of Ranvier. It was also present at this site in the central nervous system where the appearance was less characteristic than in the peripheral nervous system. The protein was also observed around about 10% of neurones in all of the regions studied. The subcellular structures labelled could not be precisely defined with the optical microscope.