ELECTRIC brain responses were measured to infrequent tones that broke the frequency alternation of two tones, deviated in duration or violated both regularities (alter-nation and constant duration). Mismatch negativity (MMN) was elicited by both simple deviants with the duration-related MMN peaking ∼130 ms later than the alternation-related MMN. The double deviant elicited two successive MMNs. Thus violation of each regularity elicited a separate MMN, whereas previous studies showed that multiple temporally separate deviations from a single repetitive standard elicit one MMN only. These results suggest that the primary function of the MMN-generating process is more closely related to maintaining the representation of auditory regularities than to deviance detectionper se.