Gelsolin is a Ca(2+)-sensitive actin filament-severing protein. To elucidate the role of gelsolin in nervous tissues, we have investigated localization and expression of gelsolin in rat CNS and PNS using biochemical and morphological methods with a polyclonal antibody against the COOH-terminal fragment of plasma gelsolin. Immunohistochemical study showed that gelsolin was specifically enriched in oligodendrocytes and Schwann cells, and was also detected in myelin sheath, especially around the Ranvier's nodes. The immunohistochemical stainings using indirect immunofluorescence, avidin- biotin-peroxidase complex, and immunogold methods were carefully confirmed by immunoblotting against the tissue homogenates. The expressional changes of gelsolin in developing brain were investigated. The protein was detectable in newborn rat brain; however, it began to increase at 8–10 d after birth and reached maximal at 20–30 d when myelinogenesis actively occurred. After this period, the protein decreased gradually, although myelin basic protein was increasing until 6 months after birth. The immunostaining of gelsolin in Schwann cells was enhanced upon regeneration of injured sciatic nerves by freezing. Immunoelectron microscopy revealed that gelsolin was present not only in the cytoplasm but also in compact myelin. Following solubilization by detergents, gelsolin in the myelin fraction could be purified using anion exchange and blue Sepharose column chromatographies. The purified protein possessed a Ca(2+)-dependent severing activity against actin filaments similar to that of cytoplasmic and plasma gelsolin. These data strongly suggest that gelsolin in nervous tissues might be involved in lamellipodial movement to wrap axons of myelin-forming cells by modulating actin polymerization.