Auditory event-related potentials (ERPs) were recorded from the scalps of persons who received repetitive sequences of sounds through earphones. Occasionally and unpredictahly. some ofthe sounds in the sequence occurred at a shorter or longer interstimulus internal (ISI) than usual. In Experiment 1, it was shown that premature noise bursts (ISI = 300 msec) elicited a large, biphasic ERP consisting of an “Ne” component at about 130 msec and a “PE” wave at about 200 msec. The NE-PE complex was several times larger than the N1-P2 waves elicited either by the regular bursts (ISI - 600 msec) or by delayed noise bursts (ISI = 600 msec) in a sequence where the regular ISI was 300 msec. In Experiment 2, the peak amplitude of the NE wave elicited by infrequent early tones was considerably increased when attention was directed toward the tones. In Experiment 3, the amplitude ofthe NE-PE, complex to infrequent early tones in a regular sequence (ISIs of 1200 msec) was either unchanged or actually increased when ISIs were shortened from 600 to 300 msec. It was proposed that the NE-PE complex belongs to a class of ERPs that are specifically elicited by a mismatch event in a repetitive stimulus sequence.