Anionic sites in the rat sciatic nerve were studied by light and electron microscopy using a fine-granular cationic colloidal iron staining method (Murakami et al., 1986). The axon, as well as the endoneurium, the epineurium and the basement membrane of Schwann cells, were all confirmed to react strongly to the cationic colloidal iron even at a pH value of 1.0-2.0. Prior hyaluronidase digestion decreased the colloidal strain of the epineurium; chondroitinase ABC weakened that of the endoneurium and the basement membrane of Schwann cells. However, as axons retained stainability with cationic colloidal iron even after combined digestion with hyaluronidase, chondroitinase ABC, heparitinase and keratanase, the authors consider sulfated glycoconjugates and not those substances which are digestible with such common enzymes. The acid groups ionized at pH 1.0 are most likely sulfate groups. Methylation deprived the axon of the reactivity to cationic colloidal iron staining, and even subsequent saponification could not recover this reactivity to its full extent. In the axon, electron microscopy revealed a deposition of colloidal iron on the external surfaces of microtubules and neurofilaments in the axoplasm and of very fine filaments connecting them. This highly negatively charged intra-axonal network could also serve toward a supportive function in maintaining the spatial distribution of microtubules either mechanically or through electrostatic repulsion or, possibly, serve as an intra-axona cation exchange reservoir.