BY amplifying the electric changes which take place in the individual nerve fibres it is possible to record the messages which pass from the sense organs to the central nervous system and from the motor nerve cells to the muscles. It is found that a series of brief potential changes travel along each nerve fibre; the changes are of fixed intensity and duration, but their frequency varies with the intensity of the excitation. Each potential change represents a nerve impulse of the typo made familiar by the classical methods of electro-physiology. Reasons are given for the belief that all nervous communication is carried out by such impulse messages, and that the impulses, like the electric changes which accompany them, are unaffected by variations in the intensity of the stimulus. The general similarity of motor and sensory messages makes it probable that the sensory nerve endings and the synaptic regions of the central nervous system work on a common plan depending on the fundamental properties of excitable cells.