We recorded somatosensory evoked magnetic fields (SEFs) over the hand area of the primary sensorimotor cortex (SMI) in 6 healthy adults in 2 sets of experiments to study interaction of afferent impulses. In experiment 1, SEFs were elicited by contralateral median nerve (MC) stimuli presented alone and 40 msec after a conditioning stimulus to the contralateral ulnar (UC), ipsilateral median (MI) or contralateral tibial (TC) nerve. N20m, P30m and P60m deflections to MC stimulation were markedly attenuated by preceding UC stimulation whereas N40m was enhanced, and a novel P80m emerged. In contrast, MI or TC stimulation did not affect the responses to MC. In experiment 2, the time course of recovery of N20m to median nerve stimuli was studied after stimulation of the adjacent ulnar and of the same median nerve. The recovery curves were similar for both conditioning stimuli with nearly full recovery of N20m at 120 msec. The results indicate marked interaction of impulses from ipsilateral median and ulnar nerves in human SMI, but no evidence was found of interaction from the two hands or from ipsilateral hand and foot.