Recent electrophysiological studies in animals using oddball stimuli have demonstrated that neurons along the auditory pathway from the inferior colliculus to the auditory cortex (AC) have a strong response to rarely presented stimuli. This phenomenon is termed stimulus-specific adaptation (SSA), which is regarded as novelty detection. However, in the medial geniculate body (MGB), it is not clear whether SSA is frequency dependent or if neurons in the MGB are sensitive to the regularity of the stimuli. In this present study, we analyzed the relationship between stimulus frequency and SSA, as well as explored regularity sensitivity using extracellular recordings in the MGBs of rats with regular and irregular oddball stimuli. It was found MGB neurons exhibited strong SSA when the pure-tone stimulus of the oddball stimulus deviated far from the characteristic frequency, even in the ventral region of the MGB, suggesting that the MGB may contribute to SSA in the primary AC. Moreover, we found the neuronal population in the MGB was sensitive to high-order sound structure, where deviant responses were smaller and standard responses were stronger for irregular oddball stimuli. We conclude that regularity detection occurs in the MGB, but in a manner distinct from the AC.