Under normal physiologic conditions, liver size is under strict regulatory control. Activin, a member of the transforming growth factor β (TGF-β) superfamily, is expressed in the intact adult liver and is an inhibitor of hepatocyte growth. However, the exact role played by endogenous activin in maintaining the size of a normal adult liver has yet to be completely examined in vivo. Here, we report the development of an adenoviral vector (AdexCAFS288) that expressed human follistatin-288, which binds to activin and neutralizes its biologic activities. AdexCAGFP, a control virus, expressed green fluorescent protein. AdexCAFS288 effectively expressed follistatin-288, as measured both in HepG2 cell lysate and conditioned medium and blocked activin signaling and its biologic functions in vitro. Intraperitoneal injection of AdexCAFS288 in vivo resulted in significant liver growth (146% of control) in intact liver of adult male rats 12 days following treatment without significant dysfunctions. The increase in liver size was attributed to increased hepatocyte proliferation, as monitored by the mitotic index. Furthermore, there was a significant correlation between serum follistatin levels and liver weight. In conclusion, our results suggest that activin plays a critical role in maintaining optimal liver size and implicates the endogenous activin system as a therapeutic target in the treatment of liver disease.