Tellurium (Te) is a naturally occurring element with many industrial uses. Microinjection of 0.3 micrograms of potassium tellurite [K(2)TeO(3)] into the endoneurial space of rat tibial nerve causes a rapidly progressing focal conduction block as measured by the disappearance of the evoked compound muscle action potential (CMAP) of the intrinsic foot muscles following stimulation proximal to the injection site. Conduction block was fully established within 6 hours and persisted for approximately 7 days, followed by the appearance of low amplitude, long latency, temporally dispersed potentials. The proximal CMAPs increased in amplitude and decreased in latency and temporal dispersion until normalization by 28 days after injection. The distal CMAP showed a minimal decline in amplitude. Morphological observations showed splitting of myelin, especially in the paranodal regions, followed by accumulation of myelin debris in Schwann cells and macrophages. Although the exact mechanism remains unknown, this in vivo model provides a unique opportunity to study the electrophysiological and morphological correlates of an acutely evolving demyelinative process.