Multiplication of pseudorabies virus (PRV) strain TOP and establishment of latency were followed in the tonsils and Gasserian ganglia (GG) of 8-week-old pigs after oral infection. Already within 24 hr, the titre of PRV in tonsils reached 10(2) TCID50/g tissue, its presence in the oropharynx being detectable by swabbing. Virus multiplication in tonsils culminated on days 3-6 p.i. (10(6)-10(6.5) TCID50/g) and continued till day 11 post-infection (p.i.), when its titre was 10(1.5) TCID50/g the swabs being still positive. The presence of the virus in GG was first proved at 48 hr p.i., but no virus could be found there by titration on day 11. The virus titre in the GG had essentially followed that in tonsils, however, it was substantially lower during the whole experiment. After primoinfection no shedding of PRV was detected by oropharyngeal swabs taken at 3-5 day intervals during the period of 360 days. Nevertheless, explantation of tonsils and GG, performed between days 60-360 p.i., revealed the presence of PRV latency in 41.1 per cent of animals. The total activation rate in GG cultures was 71.5 per cent, while in the cultures of tonsils 28.5 per cent only. Cocultivation of cell suspensions obtained by trypsinization of GG and tonsillar tissue with chick embryo cells (CEC) did not result in detection of the latent virus though latency was confirmed by explantation of the same tissue samples. However, already 24 hr in culture were sufficient for activation of latency as judged by trypsinization of the explanted tonsillar and GG fragments and their inoculation onto CEC. Estimation of the number of cells carrying PRV in the GG showed that activation occurred in one out of 10(4)-10(5) ganglion cells.