Rabies virus replication in suckling and adult mouse brains inoculated with either fixed or street virus was studied by light and electron microscopy. Chronologic study of fixed virus-infected suckling mice showed that virus budded from the plasma membrane of neuronal cell processes and perikarya prior to the development of intracytoplasmic virus particles. Nucleocapsids and virions were observed exclusively in perikarya and cellular processes of neuronal cells and, on rare exception in an astrocyte indicating high neurotropism of rabies virus regardless of the strain of virus and the age of host animal. Virus transit between contiguous neuronal cell processes and perikarya was observed in suckling and adult mouse brain after both street virus and fixed virus infection. Nucleocapsids and virions were also found in myelinated axons in addition to the unmyelinated neuronal cell processes. The implication of cell to cell spread of virus within the central nervous system in rabies virus infection is discussed.