The silent period and a rhythmic slower wave in EMG appear preceding a rapid voluntary movement (Tanii 1984). The present study was performed to investigate temporal relationships of the EMG changes preceding movement to premovement cortical potential shifts, in order to clarify whether the EMG changes are related to preparation and initiation of voluntary movement. A strong push of a hard band with the right wrist was conducted rapidly following a slightly sustained contraction. The surface EMG was detected from the right triceps brachii muscle. The EEG was recorded from C3, Cz and C4. Slowing of the EMG occurred before the movement in addition to the slower wave. The EMG slower waves were accompanied by a negative deflection of raw EEG potentials. The averaged cortical potentials preceding movement fell into 3 negative potentials. The first potential started about 1.0 sec before the EMG burst. Many of the EMG slower waves occurred in the phase of the second potential occurring 330-510 msec before the EMG burst. Premovement silent period appeared in the phase of the third potential occurring 30-60 msec before the EMG burst. Amplitude of these potentials was larger in the contralateral hemisphere than in the ipsilateral one. This asymmetry became statistically significant in the phase of the second potential. The results suggest that the EMG slower waves and the premovement silent period are associated with preparation and initiation of voluntary movement.