C-fos expression was mapped in the auditory pathways of rats, stimulated acoustically with pure tones. In the cochlear nucleus, two clusters of c-fos-like immunoreactive neurons, located respectively in the caudal part of the dorsal cochlear nucleus and in the granular cell region, did not show clear systematic shift in their position as a function of the tones frequency. On the other hand, more rostrally in the dorsal cochlear nucleus, a cluster of c-fos-like positive neurons moved progressively from dorsal to ventral for decreasing tones frequency. In the posteroventral cochlear nucleus, another cluster of c-fos-like positive neurons was observed, whose position also varied with tones frequency. Surprisingly, no or very rare c-fos-like immunoreactive neurons were present in the anteroventral cochlear nucleus and in the superior olivary complex. In the inferior colliculus, however, c-fos-like immunoreactive neurons formed clear isofrequency contours, shifting from dorsolateral to ventromedial for increasing tones frequency. In the medial geniculate body c-fos-like immunostaining was restricted to the medial and dorsal divisions while the ventral division was free of labeling. The cause of this differential labeling along the auditory pathways is at present unknown but may eventually provide clues as to physiological differences in parallel auditory pathways.