The primary sites of Aujeszky's disease virus (ADV) multiplication in intranasally (i.n.) infected pigs were found to be in the nasopharyngeal, tracheal and pulmonary regions. From the second day post infection (DPI) onward ADV invaded the central nervous system and other organs. The virus was isolated from the nasopharyngeal region for at least 2 weeks. In serum ADV was present with low levels from DPI 1 to DPI 7. In pigs vaccinated with an inactivated vaccine and then challenged the distribution of ADV was rather similar to that in non-vaccinated animals, in spite of the presence of neutralizing antibodies. The virus titres in the organs generally were lower than in non-vaccinated animals up to DPI 7. Thereafter, titre differences were no longer significant. Virus was isolated from the tonsils and the lungs for at least 2 weeks. Interferon production in vaccinated infected pigs was significantly lower than in non-vaccinated infected pigs. Though multiplication and dissemination of ADV occurred, vaccinated pigs did not show clinical symptoms of Aujeszky's disease. Traces of ADV were detected in a small percentage of white blood cells (WBC) of non-vaccinated infected pigs. ADV was isolated from the lymphocyte-enriched and polymorphnuclear leukocyte-enriched fractions, but not from the monocyte-enriched fractions, apparently on account of the small cell number. Multiplication of ADV was demonstrated in cultured WBC from some of the vaccinated and non-vaccinated infected animals.