In non-cephalic reference recordings, the scalp recorded short latency evoked potentials to median nerve stimulation in normal subjects consist of 3 positive potentials followed by a negative potential. The sources of these potentials have not been precisely defined. Therefore, these potentials were recorded in 31 patients with focal lesions of the nervous system. Recordings were evaluated for (a) the presence or absence of these potentials and (b) peak latency differences between components. The results were compared with similar data obtained on 25 normal control subjects. Only the first positive potential was recorded with stimulation ipsilateral to the lesion in one patient with unilateral C5-T1 root avulsion. This indicates that this potential arises in stimulated peripheral nerve fibers. The second potential, although not consistently recorded in normal subjects, had an abnormally prolonged interpeak latency in 2 patients with cervical cord and medullary lesions. Therefore, it seems that it arises in the central nervous system, either in spinal cord or lower brain stem. The third potential was absent in 2 patients with medullary lesions and its interpeak latency was prolonged in 2 patients with brain stem lesions. It was recorded in 3 patients with thalamic lesions in whom subsequent potentials were absent. This suggests that this potential arises primarily in brain stem pathways. The negative potential was absent in 2 patients with cerebral lesions which did not appear to involve the thalamus which suggests that it arises in the thalamocortical radiations or cerebral cortex. Short latency evoked potential abnormalities correlated more with impairment of proprioception than with disturbances in appreciation of pain and temperature.