Several different negative potentials elicited in auditory perceptual tasks make spatially and temporally overlapping contributions to the scalp-recorded event-related potential. Frequent non-target tones in a 2-stimulus oddball pitch discrimination task, when compared with the same stimuli in ignore or simple reaction time conditions, elicit a negative deflection with two peaks (NA1 and NA2) differing in their latency and topography from the exogenous N92-P156 deflections. Oddball tones, when compared with the frequent ones, elicit mismatch negativity (MMN) in both ignore and discrimination conditions; MMN displays a frontocentral-posterolateral polarity inversion. In the discrimination condition, MMN is followed by N2 and P3b; the former has a more central amplitude maximum than MMN, and no posterolateral polarity inversion. When the pitch discrimination task was made more difficult, there was no effect on NA1 or NA2, but the latency of MMN, N2, P3b, and reaction time all increased in parallel. It is hypothesized that MMN reflects the outcome of an automatic mismatch detection process, and that the subsequent processing of targets is related to the event of mismatch detection.