After complete bilateral section of the brachium of the inferior colliculus, cats cannot relearn an auditory frequency discrimination, a task which can be easily relearned after ablation of all known auditory neocortical areas and the subsequent degeneration of the thalamic auditory nuclei (the medial geniculate body and the posterior nucleus). The problem can be relearned when a small part of the brachium remains intact; the rate of relearning depends upon the anatomical locus of the remaining fibers. These results suggest that the brachium may contain fibers that do not end in the medial geniculate body and that these fibers, which probably come from the inferior colliculus and/or the nucleus of the lateral lemniscus, play an important role in the neural mechanisms underlying frequency discrimination. (This research was supported by the Office of Naval Research and the Air Force Office of Scientific Research.)