Previous results suggested that, after intranasal inoculation, mouse hepatitis virus (MHV), a neurotropic coronavirus, entered the central nervous system (CNS) via the olfactory and trigeminal nerves. To prove this hypothesis, the effect of interruption of the olfactory pathway on spread of the virus was studied using in situ hybridization. Unilateral surgical ablation of this pathway prevented spread of the virus via the olfactory tract on the side of the lesion. MHV RNA could be detected, however, at distal sites on the operated side, indicating that the virus spread via well-described circuits involving the anterior commissure from the control (intact) side of the brain. Viral transport via the trigeminal nerve was not affected by removal of the olfactory bulb, showing that the surgical procedure was specific for the olfactory pathway. These results prove conclusively that MHV gains entry to the CNS via a transneuronal route, and spreads to additional sites in the brain via known neuroanatomic pathways.