Amplitude enhancement in the NI component of auditory event-related potentials (ERPs) to alternately presented sounds has been referred as a typical example for the effect of release from neural refractoriness. We tested this hypothesis to see whether some other effects also contribute to this phenomenon. Two tones of different frequencies were presented singly or in pairs, and ERPs were recorded using monotonous (mnt) and alternating (alt) sequences of these stimuli. Comparison of the 'alt-mrt' difference waveforms recorded with single and paired stimuli supported the refractoriness hypothesis. A mismatch negativity-like wave, however, was also observed, questioning the constraint of 'at least two consecutive standards before deviant' presumed in most mismatch negativity studies. This paradigm made it possible to delineate the ERP components related to refractoriness and mismatch detection processes.