A fasciculation with an axonal origin may be followed by its own F-response. The time-lapse between these 2 potentials corresponds to the time for the antidromic wave to travel to the alpha-cell and to return to the trigger-point. This relatively stable interval may be compared with the difference between the latencies of the F- and M-responses of the same muscle evoked by distal nerve stimulation. This permits one to localise the origin of the fasciculation on the axon. This method showed that, of 201 fasciculations followed by their F-response, obtained in normal controls and in neurological patients, 97 and 89% respectively had their origin between the muscle and the stimulation site. In this series, the proportion of pathological fasciculations having an origin between the muscle and a point at half the M-latency, is similar to that found in a former study on unselected fasciculations using a collision method (Roth 1982). Comparison between these 2 studies therefore supports the concept that all or almost all fasciculations have an axonal origin.