It is difficult in man to study disease processes due to neurotropic viruses. However, certain clinical and "pathophysiological" abnormalities common to many of these diseases, such as seizures, myoclonus, or paralyses, may well be subjected to more precise study in laboratory animals artificially infected with viruses to which they are susceptible. Routes of infection can be chosen at will and the intensity to some degree controlled. It is the purpose of this and succeeding papers to describe some observations on cats following infection with Newcastle disease virus which may be relevant in interpreting analogous human disorders. Newcastle disease virus is an epizootic infection of chickens characterized by viremia and signs of involvement of the respiratory, gastrointestinal, and central nervous systems. Doyle,1in England, first isolated a filtrable virus from a completely fatal endemic in Newcastle-on-Tyne and named the infection caused by this agent Newcastle disease. It has subsequently become