Motor unit action potentials (MUPs) of the external urethral sphincter muscle during cystometry were analysed on six normal males. Electrodes were of concentric needle type having the exposed tip surface of 0.07 mm2, and the potential changes were analysed by automatic decomposition electromyography (ADEMG), for isolation and characterization of unit discharges. Number of analysed units was as follows: Two at rest, 4 at first desire to void, and 6 at maximum desire to void. Number of the recorded units and the firing rate were increased along with the degree of bladder filling. Average MUP figures as follows: Amplitude of 206 microV, 8.3 msec. duration, and 5.4 Hz firing rate at rest; 246 microV, 9.7 msec and 7.3 Hz at first desire to void; 277 microV. 9.7 msec and 7.2 Hz at maximum desire to void, respectively. It is concluded that external urethral sphincter maintains its urinary continence by excitation of the neuromuscular units, and by the increase in the firing rate of individual MUP during bladder filling.