Although segmental and dermatomal somatosensory evoked potentials have been used in the diagnosis of lumosacral radiculopathy, etc, the unilateral or bilateral normal intertrial variations of these responses have not been investigated. Without investigation of these basic parameters, determination of what constitutes a normal somatosensory-evoked potentials (SEP) is difficult. In 30 normal subjects, The results were as follows. The mean P1 latencies from superficial peroneal nerve, sural nerve, L5 dermatome, S1 dermatome were 41.4±2.2 msec, 42.2±2.3 msec, 46.8±3.1 msec, 47.1±3.2 msec. The mean N1 latencies were 50.6±2.8 msec, 51.5±2.6 msec, 56.1±3.4 msec, 56.3±3.2 msec. The mean amplitudes were 1.5±0.6 uV, 1.3±1.0 uV, 1.3±0.7 uV, 1.0±0.5 uV. The ipsilateral intertrial variations, arithmetic mean side to side differences and maximal potentials side to side differences with stimulation of the superficial peroneal nerve, sural nerve and L5 and S1 dermatomes with respect to P1 and N1 latencies and peak-to-peak amplitudes were investigated. Considerable ipsilateral intertrial variation was observed and side-to-side comparison reveals a further increase in the above measured parameters. The maximal potential side-to-side differences were even more remarkable than the side-to-side differences. We also suggest an additional parameter with which to evaluate SEPs: the maximal side-to-side latency difference.