Summary.   Previous results from our laboratory have demonstrated that type I DNA topoisomerase activity is required for the replication and gene expression of pseudorabies virus (PRV). In the present report, we further analyzed the expression of topoisomerase I in PRV-infected cells, and the western blot result showed that the expression of topoisomerase I was increased after virus infection. The increase sustained to late time of infection when the cytopathic effect was obvious and the synthesis of most host proteins was shut off by PRV. From transient expression assay, it was also found that the promoter of cellular topoisomerase I gene could be stimulated by immediate-early protein (IE180) and viral early protein 0 (EP0), and these two regulatory proteins appeared to work synergistically. Collectively, these findings provide evidence that PRV can stimulate the expression of topoisomerase I and that the stimulation is mediated at least by IE180 and EP0 proteins of PRV at the transcriptional level.