Sodium channels, which are largely localized to the nodes of Ranvier in myelinated axons, appear to form new distributions along demyelinated axons. In this study a sensitive radioimmunoassay (RIA) was used to examine the changes in the total number of sodium channels that occur in nerves experimentally demyelinated in vivo with doxorubicin (adriamycin). The results clearly illustrate the development of an increased number of sodium channels during demyelination, suggesting that this process is associated with the formation of new sodium channels.