A detectable deviation in the physical parameters of a stimulus that is presented among the multitude of standard stimuli evokes the negative deflection of an event-related potential (ERP) in the human auditory cortex (AC) named as the mismatch negativity (MMN). The MMN is likely to reflect neural activity related to sensory memory processes in the AC (Näätänen 1992). The low-affinity NMDA receptor antagonist memantine is widely used for the treatment of Alzheimer's disease in humans. Systemic injections of memantine impaired memory task performance in rats, and the impairment was demonstrated to be due to a deficit in motor performance rather than memory processing (Willmore et al. 2001). In the present study, two tones of different frequencies were presented to anesthetized rats during the oddball paradigm (randomized 96 % of standards and 4% of deviants). Either saline or a low (3 mg/kg) or high (10 mg/kg) dose of memantine (memantine hydrochloride, Sigma-Aldrich Chemie GmbH, Germany) was injected intraperitoneally. In the saline and memantine low dose conditions a MMN-like response was recorded within a 60–240- ms window after the stimulus onset. In the memantine high dose condition a MMN-like response was abolished within a 60–180-ms interval but within a 180–240-ms window the MMN- like response occurred and it had the opposite polarity compared to the saline condition. Thus, while the memantine low dose did not block the MMN-like response, the high dose changed the ERP polarity and partially blocked the MMN-like response. Näätänen, R. 1992. Attention and Brain Function. Erlbaum. New Jersey. Willmore, C.B., Bespalov, A.Y. & Beardsley, P.M. 2001. Pharmacol Biochem Behav 69, 493–502.