The changes within central somatosensory pathways accompanying reaction movements were examined by subtracting the peak latency of the major response from the cervical area (14 msec.) from that of the primary cortical response (20 msec.) and also measuring the CNV which is dependent on the direction of the subject's attention and level of arousal. These parameters were measured before (at rest), during fast and slow reaction movements, and mental readiness to act. Analysis showed that the difference in N20-N14 potential latencies is the shortest during fast reaction movements and is shorter during movement tasks than during mental readiness and intention to act. Moreover, we confirmed that the CNV amplitudes are significantly higher during fast than during slow reaction movements and also that the difference in N20-N14 potential latencies decreases as the CNV amplitude increases. Accordingly, from the present results and a series of studies conducted by others, it may be concluded that not only are the changes in the central motor area confirmed by the movement-associated cerebral potentials but also those in the central somatosensory area occur prior to reaction movements.