Intraneurally injected lysolecithin causes both segmental and paranodal demyelination. In demyelinated internodes, axonal components of nodes fragment and disappear, glial and axonal paranodal and juxtaparanodal proteins no longer cluster, and axonal Kv1.1/Kv1.2 K+ channels move from the juxtaparanodal region to appose the remaining heminodes. In paranodal demyelination, a gap separates two distinct heminodes, each of which contains the molecular components of normal nodes; paranodal and juxtaparanodal proteins are properly localized. As in normal nodes, widened nodal regions contain little or no band 4.1B. Lysolecithin also causes “unwinding” of paranodes: The spiral of Schwann cell membrane moves away from the paranodes, but the glial and axonal components of septate-like junctions remain colocalized. Thus, acute demyelination has distinct effects on the molecular organization of the nodal, paranodal, and juxtaparanodal region, reflecting altered axon–Schwann cell interactions. J. Comp. Neurol. 479:424–434, 2004. © 2004 Wiley-Liss, Inc.