Cytomegalovirus (CMV) infection of human diploid embryo fibroblasts in vitro causes a massive cell death on the 3-4th day of infection with a high primary infection coefficient (1-5 U/cell). Cytopathological effects of viral infection on the 3-4th day includes diminishes of the cell size, changes in their form, compaction of the nuclear chromatin, and disorganization and inactivation of the nucleolus. However, the early stages of the viral infection progression (the 1st-2nd day) are accompanied by unequivocal activation of rDNA (ribosomal gene) and the bulk of chromatin transcription. There are several features to support this conclusion: In the early CMV-infected cell 1) the nucleolar size is increased; 2) the number of intranucleolar foci binding the specific RNA-polymerase I transcription factor (UBF) is augmented; 3) the Ag-NOR staining is enhanced; 4) 3H-uridine incorporation to the nucleoli is activated; 5) the ultrastructure of the nucleolus is changed. Altogether, these data argue in favor of activation of rDNA transcription in human fibroblasts in vitro at the initial stages of infection.