Ion channels and their roles in nerve are becoming more widely studied as new techniques are developed, the number of identified channel types increases, and evidence for the involvement of channels in disease processes accumulates. The first seven chapters deal with ion channels and ion exchange mechanisms from widely differing viewpoints. Ritchie reviews the distribution of Na+ and K+ channels, both along axons and between axons and supporting cells, while Waxman focuses on the node of Ranvier, and other ion movements that are important there, in addition to the sodium influx responsible for action potentials. Baker and Konishi consider different K+ channels in Schwann cells that may be involved in maintaining the immediate environment of the internodal axolemma. This comprises the vast majority of the axon membrane and, as the following chapter by Reid indicates, is very far from the passive cable once assumed. Species differences are important for some aspects of nerve and channel function: the lack of fast K+ channels at mammalian, as against frog nodes (Ritchie) is the best known, but there is also a surprising difference between K+ channels reported in mouse and rabbit Schwann cells, the latter lacking inward rectification (Baker, Konishi). Konishi's elucidation of the factors affecting expression of this conductance may be relevant to the species difference. Ultimately we may have to study channel behaviour in human axons directly to be sure that the work on animal preparations is relevant to human physiology and disease, and pioneering results with this approach are described by Reid.