In the big brown bat, Eptesicus fuscus, the response properties of neurons and the cochleotopic (frequency) maps in the auditory cortex (AC) and inferior colliculus can be changed by auditory conditioning, weak focal electric stimulation of the AC, or repetitive delivery of weak, short tone bursts. The corticofugal system plays an important role in information processing and plasticity in the auditory system. Our present findings are as follows. In the AC, best frequency (BF) shifts, i.e., reorganization of a frequency map, slowly develop and reach a plateau Ϸ180 min after conditioning with tone bursts and electric-leg stimulation. The plateau lasts more than 26 h. In the inferior colliculus, on the other hand, BF shifts rapidly develop and become the largest at the end of a 30-min-long conditioning session. The shifted BFs return (i.e., recover) to normal in Ϸ180 min. The collicular BF shifts are not a consequence of the cortical BF shifts. Instead, they lead the cortical BF shifts. The collicular BF shifts evoked by conditioning are very similar to the collicular and cortical BF shifts evoked by cortical electrical stimulation. Therefore, our working hypothesis is that, during conditioning, the corticofugal system evokes subcortical BF shifts, which in turn boost cortical BF shifts. The cortical BF shifts otherwise would be very small. However, whether the cortical BF shifts are consequently boosted depends on nonauditory systems, including nonauditory sensory cortices, amygdala, basal forebrain, etc., which determine the behavioral relevance of acoustic stimuli.associative learning ͉ conditioning ͉ hearing ͉ reorganization of tonotopic map ͉ somatosensory cortex T he response properties of neurons and the sensory map in a sensory cortex and subcortical sensory nucleus can be changed by conditioning, learning of a discrimination task, or focal cortical electrical stimulation (see refs. 1-3 for reviews). In the central nucleus of the inferior colliculus (IC) of the big brown bat, Eptesicus fuscus, the best frequency (BF) of a neuron shifts toward the frequency of a conditioned tone that is followed by an unconditioned electric leg stimulus (ES l ). The BF shift of Ϸ1.5 kHz lasts up to 180 min after a 30-min-long conditioning session. Inactivation of the primary auditory cortex (AC) during the conditioning abolishes the collicular BF shift that otherwise would be evoked (4). Focal electrical stimulation of the AC evokes collicular (5, 6) and cortical BF shifts (6, 7) very similar in the amount and duration (retention) to the collicular BF shift evoked by the conditioning. These data indicate that the collicular BF shift evoked by the conditioning is produced by the corticofugal system but do not indicate whether the collicular BF shift is due to the cortical BF shift or vice versa. It has not yet been determined which is the case, because it is very difficult, if not impossible, to inactivate corticofugal fibers selectively without inactivating thalamocortical fibers. One of the aims of the present paper is to p...