Hepatocyte growth factor and transforming growth factor-α are two well-known hepatomitogens for primary hepatocyte cultures. Here we report that these two growth factors also stimulate in vivo DNA syntheses in normal, unoperated, adult rat liver after 24-hr continuous intraportal infusion. As determined by an immunohistochemical staining technique, 5-bromo-2′-deoxyuridine incorporation was increased in a dose-dependent fashion after infusion of up to 10 μg of growth factor/100 gm body weight in the rat. Stimulation of DNA synthesis was seen in the periportal area. Pretreatment using intraportal infusion of collagenase (1 U/kg body weight) for 4 hr before administration of growth factor increased the labeling by 2- to 4-fold to a labeling index range of 48% to 52%. These results suggest that collagenases and possibly other proteases are involved in making hepatocytes competent to respond to growth factors at very early stages of liver regeneration. (HEPATOLOGY 1994;19:1521–1527.)