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 uiuo gene transfer. To examine the feasibility of this strategy, pActLacZ, an expression vector composed of the LacZ gene driven by the p-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 p-actin promoter, was evaluated. Two days after in uiuo intrabiliary administration of pAGS3MO91 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 liposomemediated in uiuo gene transfer. This model will be useful for determining the immunopathological role of HCV in uiuo. (HEPATOLOGY 1995;21:746-751.) Hepatitis C virus (HCV) is a major causative agent of acute and chronic liver diseases throughout the world,' but the mechanism underlying the hepatocellular injury is not well understood. This is partly because animal models of HCV infection have been limited to the experimental inoculation of chimpanzees, which Abbreviations: HCV, hepatitis C virus; bp, base pair; X-Gal, 5-bromo-4chloro-3-indolyl-,f-D-galactoside; PCR, polymerase chain reaction; RT, reverse transcription.