Somatosensory evoked potentials (SEPs) evoked by electrical stimulation of the median nerve at the wrist were conditioned by a preceding magnetic stimulus given over the contralateral central scalp using interstimulus intervals of 0-50 msec. The conditioning stimulus had no effect on the size of sub-cortical or N20 components of the response, but caused a large increase in the P25 parietal wave. Later components (P40/N65) of the SEP were reduced in size. The optimum position for producing the effect was with the magnetic stimulator held over the best point on the scalp for evoking muscle responses in the contralateral hand. The threshold for affecting the SEP was approximately the same as the threshold for evoking EMG responses in active hand muscles. We conclude that the magnetic conditioning shock changed the excitability of somatosensory cortex neurones involved in producing the P25. The subsequent decrease in the later components of the SEP may be related to the increase in somatosensory detection thresholds produced by a conditioning magnetic shock.