Late auditory evoked potentials were recorded in eight adults according to stimulus duration (50ms versus 250ms) and to speech nature of the stimulus (tones versus syllables). The main effect of these parameters concerned the negative fronto-central wave, which occurs beyond 200ms termed N250 in this paper; it had a greater amplitude and a longer latency when tone duration increased from 50 to 250ms and a longer latency, a greater amplitude and a longer time-course in response to syllables compared to tones. Moreover, this wave was inversed in polarity at the mastoid sites, indicating the involvement of generators of the N250 wave in the supratemporal auditory cortex. These results showing the sensitivity of the N250 wave to tone duration and to speech stimuli indicated that this electrophysiological index might allow further assessment of cortical activity involved in speech stimuli processing.