The Mismatch Negativity (MMN) component of the auditory event-related potential (AERP) is elicited by infrequent, physically "deviant" stimuli in a sequence of frequent homogeneous stimuli ("standard"), for instance, by a change in frequency, intensity or duration etc. When the inter-stimulus interval (ISI) of tone pips was occasionally shortened either to 300, 150, 75, 60 and 52 ms from regular ISI 600 ms, which resulted in the elicitation of MMN. The amplitude of MMN elicited by occasional shorter ISI was not reduced as a function of ISI shortening. This indicates that the MMN is not just due to activation of new afferent elements by deviant stimulus. Thus, by no means the elicitation of the MMN can be explained on the basis of the refractoriness.