In normal subjects the short latency SEPs generally consisted of 3 positive waves (P9, P11, P14) and a succeeding negative wave (N20). To determine the origins of these waves we have made intracranial records from 17 patients, which suggest the following results. P9 originates in stimulated median nerve peripheral to the dorsal roots such as brachial plexus, P11 in the dorsal column of the cervical cord, P14 in the cuneate nucleus and medial lemniscal pathway, and N20 in the cerebral cortex. On the basis of intracranial and intraspinal records, the onset of P11 indicates the arrival of the afferent volley at the cord entry and the peak latency of P11 its arrival time at the C1-2 level dorsal column. The onset latency of P14 indicates the onset of postsynaptic events in cuneate nucleus neurons and the peak latency of P14 arrival at the midbrain. From the ventral surface of the brain-stem 3 positive waves (P'9, P'11, P'14) like the initial positive components of the scalp short latency SEPs (P9, P11, P14) were recorded. The amplitude of P'14 was large compared to that of P14. The peak latencies of P'14 recorded at the medulla and the pons were shorter than that of P14 by 0.7-0.8 msec and 0.2-0.5 msec, respectively. The peak latency of P'14 at the midbrain was almost the same as that of P14. By measuring the distance between the recording electrodes in the brain-stem and the peak latency difference of P'14, the fastest lemniscal conduction velocity was estimated as 56 m/sec.