A method, multiple point stimulation, has been reported to isolate 5-20 single hypothenar or thenar motor units for investigation. This method is attractive for testing neuromuscular transmission because the required stimulus intensities are much less than for supramaximal nerve stimulation, and movement artefact is less of a problem. In this investigation of controls, the changes in hypothenar and thenar motor unit surface voltage and latency of single muscle fibre action potentials belonging to the motor unit have been measured in response to trains of stimuli delivered to the motor nerve. In healthy motor units, increases in the surface peak-to-peak voltage and corresponding reductions in the peak-to-peak duration occurred, the changes being maximum at the shorter stimulus intervals and accompanied by increased synchronisation of motor unit muscle fibre action potential discharges. The investigations of the responses for healthy single motor units to repetitive stimulation provide the basis for investigations of neuromuscular transmission at the level of the motor unit in diseases of neuromuscular transmission.