The present study was conducted to assess the role of transforming growth factor β (TGF-β) and activin(s) in the regulation of the mass of the liver. To this end, we eliminated TGF-β or activin signaling in intact rat liver by adenovirus-mediated transfer of the gene encoding truncated type II TGF-β receptor (AdextTR) or truncated type II activin receptor (AdextAR). In intact rat liver that received a single application of either AdextTR or AdextAR via the portal vein, DNA synthesis as assessed by bromodeoxy uridine (BrdU) labeling was induced. In AdextTR- or AdextAR-treated rats, nuclear labeling was significantly higher than that in AdexLacZ, adenovirus vector encoding Escherichia coli β-galactosidase gene, or saline-treated rats at 3, 5, 7, and 9 days of infusion. The peak of the BrdU labeling was observed after 7 days of infusion and the labeling decreased thereafter. Apoptosis of hepatocytes, assessed by the terminal deoxynucleotidyl transferase (TdT)-mediated, dUTP-biotin nick-end labeling method was detected after 9 days of infusion. Immunoreactivity of TGF-β and activin A increased in the liver after the blockade of the activin or TGF-β signaling. TGF-β and activin A may have been up-regulated when the action of these ligands was blocked. These results indicate that blockade of the action of either TGF-β or activin leads to the initiation of DNA synthesis in intact liver. TGF-β and activin tonically inhibit hepatocyte growth even in intact liver and may play a critical role in the maintenance of constant liver mass.