The lack of a small animal model of hepatitis C virus (HCV) infection has impeded elucidation of the pathogenesis of HCV. The aim of this study was to develop an HCV-expressing animal model by means of cationic liposome-mediated in vivo gene transfer. To examine the feasibility of this strategy, pActLacZ, an expression vector composed of the LacZ gene driven by the beta-actin promoter, complexed with lipofectin, was injected retrogradely into the common bile ducts of adult rats. X-Gal histochemical staining clearly showed that the LacZ gene was expressed in hepatocytes, but not in biliary epithelial cells. Maximal expression was observed at a DNA to lipofectin ratio of 1:4. Based on this observation, pAGS3M091, an expression vector containing the full length of HCV complementary DNA (cDNA) preceded by the beta-actin promoter, was evaluated. Two days after in vivo intrabiliary administration of pAGS3M091 complexed with lipofectin, polymerase chain reaction (PCR) amplification of reverse-transcribed liver RNA demonstrated the 5' and 3' portions of HCV transcripts derived from pAGS3M091. Immunohistochemical analysis showed the HCV core protein in a small number of hepatocytes scattered in the hepatic lobules. We conclude that the full-length HCV genome was successfully expressed in adult rat liver by means of cationic liposome-mediated in vivo gene transfer. This model will be useful for determining the immunopathological role of HCV in vivo.