Cats with chronically implanted electrodes were used to study changes in the auditory pathway during the wake/sleep cycle. Evoked responses to electrical stimulation of brain stem and thalamic nuclei were recorded and the animals' state of arousal was determined by observation and by monitoring EEG and EMG. Evoked responses in superior olive and inferior colliculus were invariant during the wake/sleep cycle. Evoked responses in medial geniculate and auditory radiations showed significant changes in slow wave sleep compared with wakefulness when stimulated from nuclei below inferior colliculus. However, with inferior colliculus stimulation, no significant changes in response at medial geniculate or radiations were observed. Evoked responses recorded from posterior A1 cortical sites showed increased amplitude of the late wave and simplification of the early biphasic waves during slow wave sleep. In contrast, evoked responses recorded from Ep and anterior A1 sites decreased in amplitude or were absent during slow wave sleep compared to responses recorded during wakefulness. No significant differences between evoked responses recorded at cortex during wake or REM sleep were obtained. It is suggested that in the auditory system, the major changes occurring during the wake/sleep cycle are cortical in origin, and sub-areas of auditory cortex change differentially. Some change is also observed in medial geniculate only if stimulation is below the level of inferior colliculus. © 1971.