Non suppurative periventricular encephalitis and lymphoid infiltration of choroid plexus were induced by intracerebral inoculation of non neuroadapted strains of mumps virus in cercopithecus monkeys. Inflammatory lymphoid infiltration showed an obvious connection with a mild damage of ependymal cells. No signs of neuronal involvement and no inclusions were seen in light microscopy. Proliferation of viral microtubules, though without the typical budding of virions, were visible in ependymal cells in electron microscopy. Subependymal and inflammatory cells which clearly take no part in the virus proliferation were affected by the alteration.