Neuroid conduction, as here defined, refers to the propagation of electrical events in the membranes of non-nervous, nonmuscular cells. Examples from protistants, plants, and animals are described. In the dinoflagellate Noctiluca, propagated membrane depolarizations accompany the spread of the luminescent response, and local electrical changes are associated with tentacle movement. The ability of Noctiluca to conduct is attributable to its peculiar geometry, in which much of the cytoplasm is confined to a thin peripheral layer surrounding the flotation vacuole, a condition that permits local current flow to develop. Fresh-water algae of the family Characeae show propagated action potentials in which chloride efflux is the major event responsible for electrogenesis. Chloride spikes are presumed to be an adaptation to fresh water, following Grundfest (1966c). Among the higher plants, Dionaea and Mimosa provide examples of neuroid conduction. The former also possess trigger cells which function in an analago...