SummaryIpsilateral and contralateral SERs to stimulation of the median nerve have been studied in man. Contralateral responses with distinct peaks were observed constantly in every control subject and ipsilateral responses were also obtained. The latencies of the ipsilateral responses were longer than those of the contralateral ones except initial small positive peak. The amplitudes of the ipsilateral responses were smaller than those of the contralateral ones.In patients with sensory impairment, the normal ipsilateral and contralateral responses appeared when the non‐affected side was stimulated. When the affected side was stimulated, however, the ipsilateral and contralateral SERs were markedly reduced or abolished. An autopsy case with metastatic tumor of the one thalamus was reported. In this case, there was no apparent response over the ipsilateral and contralateral hemisphere after stimulation of the affected side,These results are compatible with the idea that ipsilateral SERs are the result of subsequent activation of the ipsilateral hemisphere via an interhemispheric pathway, possibly involving the corpus callosu