The present study was conducted to assess the role of activin(s) in the regulatory mechanism to maintain constant liver mass. To this end, we infused follistatin, an activin antagonist, into the portal vein of the rat. Follistatin induced DNA synthesis, as assessed by bromodeoxy uridine labeling, in intact livers. Small peaks of bromodeoxy uridine labeling were observed after 3 and 18 hours of infusion, and a large peak was observed after 48 hours. In follistatin-treated rats, the DNA content of the liver was significantly elevated after 72 hours and returned to the basal value within 120 hours. Likewise, liver weight increased significantly after 60 and 72 hours, but returned to the control value within 120 hours. Apoptosis of hepatocytes, assessed by the Tdt-mediated, dUTP-biotin nick end labeling method was observed after 72 hours or later. Messenger RNA (mRNA) expression of hepatocyte growth factor, transforming growth factor-alpha, tumor necrosis factor-alpha, and interleukin-6 did not increase after the addition of follistatin. The mRNA expression and immunoreativity of transforming growth factor-beta increased after the administration of follistatin. These results suggest that the blockade of activin action leads to the initiation of DNA synthesis in the intact liver. Activins may tonically inhibit hepatocyte growth in the intact liver. Transforming growth factor-beta may also act to maintain constant liver mass when activin action is blocked.