Objective The medial geniculate body (MGB) and inferior colliculus (IC) are two important stages in the auditory ascending pathway.In order to study their functional roles in the relaying pathway, we recorded simultaneously the neuronal responses of the two nuclei in behavioral rats under different physiological conditions.Methods (1) Teflon-coated tungsten electrodes were chronically implanted into the MGB and IC to record the spontaneous activity and the responses to sound stimuli.(2) The electroencephalogram and eleetromyogram were recorded to define the different stages of sleep, awake and anaesthetic states.(3) Histology of brain slides was used to confirm the location of the electrodes.Results (1) Both MGB and IC neurons showed low spontaneous (tonic) activity in slow wave sleep (SWS) and under anaesthesia and high spontaneous activity in the awake state.In rapid eye movement (REM) sleep stage, the spontaneous activities of these two nuclei were also lower than that in awake state, but higher than that in SWS stage.(2) In the MGB, the phasic responses to white noise stimulation increased as the sleep going to SWS.In the IC, the phasic activity conversely decreased as the sleep changed to the REM stage.Conclusion The differences in the auditory responses of the MGB and IC in different physiological conditions would imply the differential modulations of the corticofugal projections to the two nuclei.