Investigation of the routes of infection of viral labyrinthitis was undertaken using mouse cytomegalovirus (CMV) and CMV-free Swiss-Webster mice. Intraperitoneal and intranasal inoculation caused no CMV infection of the ears or central nervous system. Intraperitoneal inoculation of pregnant mice yielded no evidence of fetal infection. Intracerebral inoculation of newborn mice resulted in viral infection of the ear by extension: 1) from the arachnoid into cochlear periotic connective tissue via the cochlear aqueduct and 2) along the perineurium of the acoustic nerve into the modiolus. There were neither changes in the stria vascularis nor endolymphatic labyrinth in spite of vasculitis and viremia. This experimental labyrinthitis resembles that found in human temporal bones infected with herpes zoster rather than the endolymphatic labyrinthitis seen in rubella, rubeola, mumps, and cytomegalovirus. The chronic effects of the mouse CMV labyrinthitis are under study.