Despite intensive efforts in the treatment of hepatocellular carcinoma (HCC), its prognosis remains poor, mainly because of intrahepatic metastasis. It is, therefore, important to investigate the invasive and metastatic behavior of HCC. To examine this, murine HCC cells were labeled with the fluorescent carbocyanine dye, DiI and implanted under the capsule of the liver of syngeneic mice. Optimal conditions are described for labeling HCC cells with DiI. Histological analysis using fluorescent and confocal microscopes revealed that HCC cells migrate to and invade the adjacent portal vein, but not the adjacent central vein. Conversely, DiI-labeled hepatocytes were shown not to migrate in the liver. These results suggest that intrahepatic metastasis of HCC occurs by invading the portal venous system. Furthermore, it is indicated that orthotopic implantation of fluorescent dye-labeled tumor cells may be a convenient and useful method to investigate the invasive and metastatic behavior of various types of cancer.