The purpose of this experiment was to examine event-related potentials (ERPs) recoreded from the rat's hippocampus and cerebellum during a passive auditory oddball task. It has beeen predicted that mismatch negativity like (MMN-like) and P3-like components are elicited by the infrequent stimuli in the hippocampus. Further it was questioned that whether MMN-like and P3-like components can be observed at the cerebellum. ERPs recorded from those regions consisted of P1,N1,P2,N2,and P3-like components. P3-like component was obscurely elicited by infrequent stimuli at the hippocampus and the cerebellum. In both regions, a significant difference in N2 amplitude between infrequent and frequent stimuli was observed. N2 component of the rat may correspond with MMN of the human. No significant differences of these components amplitude between infrequent and frequent stimuli were found for other components. N1 latency at the cerebellum was significantly shorter than those in the hippocampus. Correlation analysis revealed that the maximal correlation coefficient between the cerebellum and the hippocampus was obtained at -20.0ms (max : -56,min : -7). This finding suggests that information processing in the cerebellum may be carried out earlier than that in the hippocampus.