An animal model of encephalitis B was successfully established with Jin Wei Yan I strain virus in mice,which made it possible to study the ultrastructural pathology as well as the morphogenesis and release of encephalitis B exerimentally.The ultrapathological changes occurring to the brain tissues especially in the neuron cells were described systematically.A preliminary hypothesis about the morphogenesis and release of encephalitis B virus (Jin Wei Yah I strain) was proposed on the basis of these changes.Replication of the virus was also found to occur in the oligodendroglia but not in the microglia,which maintained its phagocytic function.This fact evidently shows that the micrioglia is not controlled by the mRNA of the virus.According to the literature available to the authors,it seems that the ultrastructural pathological changes of encephalitis B with typical histological lesions in animals had not been reported previously.