Clinical, virological, morphological, and immunofluorescence investigations were carried out on 22 pigs experimentally infected with two strains--a virulent one (V) of a 10(-3) titer, and a slightly virulent one (K) of a 10(-7) titer--of the virus of Aujeszky's disease as well as on contact pigs. Results revealed variations in the clinical and morphological manifestation of the disease in the individual groups. They were shown to be due to the strains of the virus that varied in virulence and tissue tropism. In the strain B-infected animals there were clinical symptoms characteristic of the disease, and the morphological changes in the central nervous system were of the nonsuppurative encephalitis type. The pigs infected with the K strain of the virus showed no clinical symptoms, while the contact animals manifested only respiratory troubles as well as interstitial pneumonia. It was demonstrated that the strains of slight virulence had weak nervotropic and pronounced pneumotropic characters. Complex virologic, morphologic and immunofluorescence investigations can be used for the diagnosis of the atypical forms of Aujeszky's disease and for the differentiation of the virulent from the slightly virulent strains of the virus.