Several genes responding to trans-synaptic stimulation have been identified in neurons. The most common genes of them is proto-oncogene c-fos, which is induced by various kinds of stimulations such as noxious, physiologic and electrical stimulation. The expression of Fos, a protein product of proto-oncogene c-fos, has been studied as a potential cellular marker of neuronal activity. Recent studies have shown that Fos is rapidly and transiently expressed in the neurons of the auditory brainstem nuclei with audiogenic shock and acoustic stimulations. The present study was performed to determine if Fos expression could be induced in the auditory brainstem nuclei following unilateral pure tone stimulation and to identify the expression patterns. Adult Sprague-Dawley rats were exposed to 90dB, 4KHz pure tone unilaterally for 60 minutes and expression of Fos was detected by immunoperoxidase immunohistochemistry using polyclonal anti c-fos antibody(Oncogene Science Co.). Fos-like immunoreactive staining was observed in ipsilateral cochlear nucleus and medial trapezoid body. In the inferior colliculus, intense Fos immunoreactivity was seen in neurons of contralateral side of stimulation, whereas much less immunoreactive neurons were seen in ipsilateral side. Thus, Fos expression may provide a useful method of investigating neuroanatomical relationships and signal processing in the auditory brainstem nuclei following acoustic stimulation.