Neuronal responses to auditory stimuli and electrical stimulation were examined in 104 neurones in the auditory sector of thalamic reticular nucleus (TRN) and 9 medial geniculate (MGB) neurones from anesthetized guinea pigs. TRN neurones showed rhythmic spontaneous activities. TRN neurones changed firing pattern over time, from tonic to burst in a time interval of several seconds to tens of seconds. One third of the TRN neurones (25/76) responded to the acoustic stimulus in a slow oscillation mode, either producing a spike burst at one time and responded with nothing another time, or producing a spike burst at one time and a single spike at the other. Thirty-two of 40 neurones received a corticofugal modulation effect. Nineteen of thirty-two neurones responded directly to electrical stimulation of the cortex with an oscillation of the same rhythm (7-14 Hz) as its auditory-evoked oscillation. Six neurones changed their firing pattern from burst to tonic when the auditory cortex was activated. As the TRN applied inhibition to the MGB, the oscillatory nature of inhibition would affect the fidelity of MGB relays. Thus, it was unlikely that the MGB was in relay mode when the TRN was in a slow oscillation mode. These results hint at a possible mechanism for the modulation of states of vigilance through the corticofugal pathway via the TRN.