Hepatocyte growth factor (HGF) is the most potent mitogen of mature hepatocytes in primary culture, and is a molecule composed of 69 kD alpha-chain and 34 kD beta-chain. HGF predominantly acts on various epithelial cells as a mitogen, motogen and a morphogen. HGF mRNA and HGF protein increases rapidly in the liver and plasma of rats with liver injury such as hepatitis, ischemia, physical crush and partial hepatectomy. Production of HGF in the liver occurs in Kupffer cells, sinusoidal endothelial cells, and Ito cells, but not in hepatocytes. HGF mRNA is also rapidly increased in the intact organs such as lung, kidney and spleen. Thus, HGF may act as a hepatotrophic factor for liver regeneration through two mechanisms: a paracrine mechanism and an endocrine mechanism. Moreover, intravenously injected HGF enhances liver regeneration and protects hepatitis in vivo. Consequently, HGF may prove to be useful for the clinical treatment of patients with liver disease. Recently, we found a factor which specially appears in the blood of rats with organ injury and increases the synthesis of HGF, and it was named "injurin". IL-1 alpha and IL-1 beta are also positive regulators for the expression of the HGF gene, while TGF-beta and Dexamethazone down-regulate HGF expression.