In addition to its mitogenic, motogenic and morphogenic functions on various cell types, hepatocyte growth factor (HGF) also suppresses mitosis in several cancer cell lines, including carcinomas and sarcomas. Here we report that HGF is also mito-inhibitory in rat liver tumors induced by diethylnitrosamine. By using a double labeling technique employing [3H]thymidine and 5-bromo-2'-deoxyuridine to determine cell proliferation before and after HGF infusion, we determined that continuous infusion of 20 micrograms total HGF inhibited tumor cell proliferation by 50%. The labeling in non-tumor areas showed the reverse result in that the labeling was higher in HGF-treated rats than control rats. These results indicate that HGF has different effects on growth of normal and tumorous hepatocytes in vivo. These findings may be of relevance in understanding the role of HGF in hepatocarcinogenesis and provide added modalities for controlling growth of hepatocellular carcinomas.