Somatosensory evoked potentials (SEPs) recorded over the hemisphere ipsilateral and contralateral to the electrically stimulated median nerve were analysed in the presence or absence of interfering movement, tactile and vibratory activity in the contralateral hand. In the control wave forms the early potentials recorded over the ipsilateral frontal region, P19 and N25, showed apparent phase reversal with N19 and P25 respectively in the contralateral parietal region and were therefore considered to be produced by the same generator in the primary sensory cortex. All other potentials recorded over the ipsilateral hemisphere were of the same polarity and similar latency to potentials present contralaterally, and their amplitude diminished progressively with distance from the contralateral hemisphere. They were therefore believed to be volume-conducted from the contralateral hemisphere, rather than originating in the ipsilateral hemisphere through uncrossed ascending pathways or interhemispheric connections. The distribution of effects produced by interfering activity in the contralateral hand was demonstrated by subtracting the 'interference' from the 'control' response to derive a 'difference' wave form. Difference potentials present over the contralateral scalp were consistent with influence on generators in the contralateral hemisphere. In particular, voluntary movement of the contralateral hand resulted in difference potentials distributed in accordance with a proposed generator in area 3a or 4. No consistent difference potentials were found in the ipsilateral parietal region. In the ipsilateral frontal region, however, an upward potential whose latency was around 55 msec was proposed to be due to a generator in the supplementary motor area of the contralateral hemisphere.