THE primary function of the nerve cell is to process electrical signals. Over the past fifteen years(1) there has beers renewed Interest in the detailed spatial analysis of signalling in individual neurons owing to experimental evidence that the regional electrical properties of neurons are complex. Thus the behaviour of many nerve cells cannot be understood on the basis of microelectrode measurements from the soma. Regional electrical properties of neurons have been studied using sharp microelectrode(1) and patch-electrode(2,3) recordings from neuronal processes, high-resolution multisite optical recordings of Ca2+ concentration changes(4,5), and by using models to predict the distribution of membrane potential in the entire neoronal arborization(6). Additional direct evidence about electrical signalling in neuronal processes in situ tan now he obtained by recording membrane potential changes using voltage-sensitive dyes(7,21). Here I demonstrate the existence of multiple action potential trigger zones in separate regions of the neuronal arborization of an indentified molluscan neuron.