Results are presented of studies on the various localization of the virus of Aujeszky's disease (the avirulent mutant strain MK and the virulent strain 2) in the organism of experimentally infected pigs. At the oral infection of pigs with strain MK the virus has been isolated from the submandibular lymph nodes of the animals only, the pigs being killed on the second and fifth day following infection with a low plaque-forming titer. However, from pigs that have been infected subcutaneously no virus was isolated. In experiments with the virulent strain 2, 15 out of 20 pigs have died up to the seventh day following infection. The examination has revealed that a virus of a high plaque-forming titer was present in almost all organs. Histologic investigations in the experiments with the virulent strain 2 have revealed the presence of a strongly manifested nonsuppurative encephalitis, interstitial pneumonia, necroses, and nuclear inclusion bodies. In the experiments with strain MK these changes have been much more slightly expressed, and after the tenth day of infection they tended to disappear. In the authors' opinion the changes are due to the 'interaction' between the viral particles of the vaccinal virus and the competent cells, being the cytologic manifestation of the enhanced immunobiologic status of the host organism. This is an expression of the cell manifestation of immunogenesis and not of the toxic injury of cells which has been observed in experiments with the virulent strain.