To evaluate species differences in the serotonergic innervation of the superior olivary complex, serotonergic fibers and varicosities were labeled with immunohistochemistry in mouse. Many immunoreactive fibers and varicosities were observed in two of the three principal nuclei, in addition to some periolivary nuclei. This pattern of staining differs greatly from that observed in other mammals in which periolivary, but not principal nuclei are richly innervated by serotonin (5-HT). These results indicate a functional relationship between the 5-HT system and both the ascending and descending auditory systems in the mouse.