The present experiment was carried out to study the pathogenesis of Newcastle disease(ND), ND virus (NDV) antigens and genes in various organs from NDV inoculated chickens were detected by immunohistochemistry and RT-PCR. Immunohistochemically, NDV antigens were detected in the spleen, thymus, cecal tonsil, proventriculus, trachea and lungs at 12 hour post-inoculation (hpi). Viral antigens were localized mainly in the cytoplasm of lymphocytes and macrophages. After 48 hpi, clinical findings of the affected chickens were open-mouth breathing, conjunctivitis, watery diarrhea and edema around the eye and neck. After 72 hpi, chickens showed muscular tremor, paralysis of the legs and wings, and coma. Histopathological results consist of multi-focal necrosis with hemorrhages in lymphoid aggregates of the intestinal tracts, necrosis of the lymphoid tissues, neuronal degeneration and necrosis, and perivascular cuffing. Using RT-PCR, virus genes were detected in the spleen and proventriculus at 48 hpi, and in the brain at 60 hpi.