The conduction velocity in muscle in situ has been measured directly by means of needle-electrodes. In the present study, the authors tried to develop a method for the estimation of conduction velocity with surface-electrodes. The subjects were four healthy adult males, aged 23 to 29 years. EMGs were led off from m. biceps brachii by several surface-electrodes and a bipolar fine-wire electrode during isometric weak contraction with the arm bent at right angles. Seven or eight surface-electrodes (8mm in diameter) were placed on the belly along the muscle (interelectrode distance was 20mm). A bipolar fine-wire electrode was inserted in the middle of the muscle. (Fig. 1) The EMGs recorded on the magnetic tape were analyzed through a mini-computer system (ATAC-2300). By triggering a signal averager from spike potentials of single motor units recorded by the inserted electrode, the average contribution of single units to the surface EMG could be extracted. (Fig. 3) The conduction velocity in m. biceps brachii, determined from the averaged waves of each surface-electrode was 4.6±0.5m/s. Also the conduction velocity estimated from the cross-correlation analysis between the two EMGs led off by two different surface-electrodes was 4.4m/s.