1. Sodium channel current density was measured using the loose-patch voltage clamp technique. Innervated rat, mouse and snake muscle had the highest density of Na+ channels in the end-plate region. These high Na+ channel densities were maintained in denervated muscle. 2. Perijunctional membrane had a Na+ current density 5- to 10-fold greater than the density several hundred micrometres from the end-plate. In all muscles this concentration of channels near the end-plate persisted following denervation. 3. At the tendon Na+ current density fell to low values (approximately 1 mA/cm2). The decrease in density began about 300-500 microns from the tendon. This pattern was found in all snake twitch fibres and fast-twitch (EDL) rat and mouse muscle fibres. This reduction in channel density near the tendon was not affected by denervation. 4. Sodium channels in all regions of innervated rat and snake muscle fibres were highly sensitive to tetrodotoxin (TTX). Sodium channels in snake muscle remained sensitive to TTX after denervation. Sodium channels that are relatively resistant to TTX appeared in rat muscle after denervation. TTX-resistant channels were even more concentrated near the end-plate than were TTX-sensitive channels in innervated muscle. At the tendon TTX-resistant Na+ channel density decreased. 5. We conclude that although the nerve presumably directs the localization of Na+ channels during development, the ability to maintain this distribution and to control the distribution of newly appearing channels persists long after the nerve has been removed.