We examined the hepatotrophic activity of heparin-binding EGF-like growth factor (HB-EGF), a recently identified potent mitogen for vascular smooth muscle cells and fibroblasts. HB-EGF stimulated DNA synthesis of rat hepatocytes in primary culture in a dose-dependent manner up to 30 ng/ml. The maximal stimulation by HB-EGF represented more than 80% of that induced by HGF. In normal rat liver, the transcript of HB-EGF gene was detected in the non-parenchymal cells and very low level in the hepatocytes. In the regenerating liver on the 3rd day after 70% hepatectomy, the HB-EGF mRNA increased in the non-parenchymal cells, suggesting that HB-EGF may contribute to liver regeneration through a paracrine mechanism.