Nine pigs were examined for the presence of viremia during the first week after oronasal inoculation of 10(8.0) TCID50 Aujeszky's disease virus (ADV). Blood was taken at 1, 2, 3, 5 and 7 days post inoculation (PI) and the presence of cell-free ADV in plasma and of ADV-infected mononuclear cells was examined by titration and by cocultivation with permissive cells, respectively. The mononuclear cells of 6 of the 9 pigs, collected at 3 and 5 days PI were further separated into subpopulations of enriched monocytes and enriched lymphocytes. Both subpopulations were cocultivated. Nasal secretions were collected from 4 of the 9 pigs for the determination of virus titers and interferon concentrations. Both infected mononuclear cells and cell-free ADV were demonstrated in 5 pigs, infected mononuclear cells only were found in 2 pigs, and neither cell-associated or cell-free ADV were detected in 2 pigs. Two of the 7 viremic animals were positive on one single day, 3 on 2 days, 1 on 3 days and 1 on 4 days. The number of infected cells was approximately 5 times higher in monocytes than in lymphocytes. The highest virus titers were present in those nasal fluids with the lowest alpha-interferon concentration. A correlation between the titer of locally produced ADV in the nose and the presence of a viremia was not found. In conclusion, we can state that a viremia regularly occurs under both cell-free and cell-associated form after an oronasal inoculation of ADV and that monocytes are the most susceptible mononuclear cells.