The tetracycline-controlled transactivator (tTA)-mediated gene activation system was examined in virus infected cells to determine its role in the control of gene expression. In the presence of tTA, the gene expression from the tetO-modified minimal promoter was efficiently activated in the uninfected cells, whereas essentially no activation was observed from the only minimal promoter without the seven direct repeats of 42 bp tetO sequences. However, essentially no activation was observed when only the minimal promoter was used, without the seven direct repetitions of the 42 bp tetO sequences. On the other hand, in the infected cells, a substantial background of $eta$-glucuronidase expression was detected in the absence of tTA, even though tTA stimulated the gene expression by ~7-fold. This background expression indicates that the sequences within or nearby tetO are involved in the background stimulation of the gene expression by HCMV and HSV-1 . These results suggest that the application of the tTA-mediated gene activation system may not be extremely useful for studying the biological roles of HCMV and HSV genes In the viral replicative cycles, because of the basal activity of the gene expression.