Following intranasal instillation of Sendai virus in newborn mice an extensive virus infection of respiratory epithelium and olfactory mucosa was observed by immunoperoxidase technique. Viral antigen appeared in olfactory nerves and in neurons of the trigeminal ganglia. Selective labeling of neurons in trigeminal ganglia was also seen after virus injection into the snout. This shows that respiratory infections may not be restricted to the respiratory mucosa but may also spread to peripheral ganglia after uptake of virus at axonal terminals and somatopetal axonal transfer to the nerve cell bodies. Following intracerebral injection into newborn mice viral antigen persisted in scattered neurons in the thalamus and mesencephalon after 24 weeks. The majority of the mice developed hydrocephalus, for which obliteration of the aqueduct seems to have been a major cause.