When 1 microgram 125I-follistatin was administered into a rat intravenously, radioactivity levels in serum decreased rapidly. Analysis with a biexponential equation showed that the initial half-life and the terminal half-life were 4.0 and 130.8 minutes, respectively. After 2 hours of infusion, approximately 9% of the follistatin infused remained in the liver, which was much more than that in kidney, spleen, pancreas, intestine, or lung. Autoradiography of the liver obtained at 24 hours of infusion revealed that numerous grains were located in parenchymal cells. Radioactivity of 125I-follistatin in the liver remained elevated until 72 hours and declined markedly thereafter. When a booster shot of 125I-follistatin was administered at 72 hours, radioactivity in the liver at 120 hours was markedly increased compared with that in rats that received a single shot of 125I-follistatin. We then examined the effect of intravenous infusion of follistatin on liver regeneration after hepatectomy of 70%. Immediately after the hepatectomy, either 1 microgram follistatin or saline was infused intravenously. In some rats, a booster shot was infused at 72 hours. After 120 hours of hepatectomy of 70%, remnant liver weight, liver regeneration rate, and DNA content were significantly (P < .05) higher in rats that received a booster shot of follistatin at 72 hours than those in control rats. These results indicate that follistatin administered intravenously accumulates in the liver and promotes liver regeneration after partial hepatectomy.