Abstract Background: Electrophysiological and behavioral studies have shown that stimulus relevance contributes to auditory processing in sleep and auditory stimuli changes the sleep stages. So we observed changes in auditory processing due to sleep stages by recording infant mismatch negativity (MMN) during different states and investigated the arousal mechanisms. Methods: Auditory event-related potentials (ERPs) of 26 neonates were recorded using high-density EGI EEG system. Stimuli consisted of 1000 Hz tones with 90% probability as standard and 1200 Hz with 10% probability as deviant. Study 1 was designed for the confirmation of the recording of MMN from neonates and Study 2 for investigating whether an appropriate stimulus onset asynchrony (SOA) of the stimulus would induce a clear difference in the latency or amplitude. Results: (Study 1) MMN were obtained from all subjects. No differences of the latencies, amplitudes and distribution due to arousal or sleep stage were observed. After the MMN response occurred, a prominent negativity like Nc was seen in response to deviant stimuli in active sleep and waking state. (Study 2) No distinct differences between the difference states were seen in any SOA. Conclusions: Only MMN did not characterize the arousal or sleep stage. But the modality of the auditory evoked potentials (AEPs) may differ according to the state, so further detailed investigation could enable the detection of the infants' state using the AEP.