Although growth factor effects have been studied in cultured hepatocytes, little information exists as to whether these factors can trigger hepatocyte replication in vivo. In this study we infused epidermal growth factor, transforming growth factor-α and hepatocyte growth factor directly into the portal vein of rats for 24 hr to see whether they could induce DNA synthesis in normal livers or in livers subjected to one-third hepatectomy. Infusion of transforming growth factor-α or epidermal growth factor at doses up to 80 μg/24 hr had little effect on hepatic DNA synthesis in normal liver, whereas the monomeric and heterodimeric forms of hepatocyte growth factor generally produced increases of less than threefold in hepatic DNA synthesis. In contrast, after one-third hepatectomy infusion of epidermal growth factor, transforming growth factor-α or hepatocyte growth factor produced dose-dependent increases in hepatic DNA synthesis. At a dose of 40 μg/24 hr, epidermal growth factor increased DNA synthesis threefold, whereas transforming growth factor-α or hepatocyte growth factor increased DNA synthesis to greater than six times that in rats that had undergone hepatectomy alone. Furthermore, infusion of these growth factors, with or without one third-hepatectomy, induced the expression of transforming growth factor-α mRNA in the liver. The pattern of protooncogene expression induced by one-third hepatectomy was studied to determine the effect of this procedure in sensitizing the liver to the growth factors. Compared with the well-characterized two-thirds hepatectomy system, there was a similar but smaller increase in c-myc expression but no induction of c-jun expression. The results suggest that a small functional deficit in the liver that by itself causes little DNA synthesis “primes” hepatocytes for replication and that the loss of liver mass and growth factor infusions complement each other by providing essential stimuli needed for DNA synthesis. (Hepatology 1994;14:489–497).