The degree to which maternally derived antibodies may affect neural invasion of Aujeszky's disease virus (ADV) in neonatal pigs was examined. One-week-old pigs with different levels of maternal immunity were inoculated intranasally with 10(7.0) TCID50 of the Ka strain. The invasion of the virus was studied in both the trigeminal neural pathway (nasal mucosa, trigeminal ganglion = 1st level, pons/medulla = 2nd level and cerebellum/thalamus = 3rd level) and the olfactory neural pathway (olfactory mucosa = 1st level, olfactory bulb = 2nd level and lateral olfactory gyrus = 3rd level) by virus titration and immunohistochemistry (IHC). In control pigs without specific antibodies, virus invaded all neuronal levels in both neural pathways. In pigs with a low concentration of maternal antibodies (SN-titer = 2-3), virus infected all neuronal levels in both neural pathways but, compared to the controls, virus titers were significantly lower (approximately 2 log10) in the trigeminal pathway. In pigs with a high concentration of maternal antibodies (SN-titer = 272-384), virus reached the 2nd neuronal level of the olfactory pathway while no neural tissue had been infected in the trigeminal pathway. Virus titers in the affected neuronal levels of the latter pigs were significantly lower than in the controls. IHC revealed, in non-immune pigs, a fibroblast-mediated spread of the virus in the nasal lamina propria, and a local spread of the virus from neurons to their satellite cells in the trigeminal ganglion. Such a spread of the virus was rarely seen in the nasal mucosa and in the trigeminal ganglion of passively immune pigs. These findings suggest that, in the presence of maternal immunity, defence mechanisms operate at these sites. In conclusion, we can state that a correlation exists between the level of maternal immunity and the protection against invasion of ADV in the nervous system of neonatal pigs.