The cytomegalovirus (CMV) major immediate-early promoter/enhancer is active in many cell culture systems and is considered to be one of the strongest promoters in vitro. However, when this promoter was used in in vivo approaches to gene therapy, it was silenced within a few weeks in several organs including the liver. In this study, we demonstrated transcriptional inactivation of the CMV promoter in mouse liver. In contrast to the CMV promoter, a hybrid promoter consisting of a minimal CMV promoter and the enhancer II of hepatitis B virus was active for at least 11 weeks in mouse liver. While investigating the reason for the shutdown of the CMV promoter, we did not find evidence for methylation of adenovirus DNA in the region of transgene insertion, but we could show that the silenced CMV promoter was reactivated after lipopolysaccharide treatment of mice or partial hepatectomy. Both stimuli are known to activate the transcription factor NFkappaB, which binds to four sites in the CMV promoter/enhancer. We show that expression from the CMV promoter in hepatocyte-derived cell lines in vitro depends on NFkappaB. In vivo experiments demonstrate that NFkappaB, which is not present in mouse hepatocytes in vivo, is activated after infection with recombinant adenoviruses and that the time course of NFkappaB activation parallels that of CMV promoter-dependent expression. Moreover, adenovirus infection of transgenic mice carrying a CMV promoter-driven lacZ gene leads to strong activation of the expression of this gene in the liver. Thus, NFkappaB is involved in the activation of the CMV promoter in the liver.