Development of the bile duct system of the mouse embryo was studied histologically and by an immunofluorescent technique. The hepatic primordium consisted of cranial and caudal portions. In the liver of young embryos, the hepatic cords were present in the presumptive cysticduct epithelium, and the histology of the presumptive cystic duct epithelium near the hilus was similar to that of the hilus epithelium. The results suggest that at least a part of the cystic duct epithelium develops from the cranial diverticulum of the hepatic primordium. Lumen structures were precursors of intrahepatic bile ducts and originated from type I (immature) hepatocytes. The lumina of the lumen structures appeared near the hilus area first, but most were discontinuous with those of the hepatic ducts. With the progress of development, the discontinuous lumen structures became distributed around the portal vein branches in the central part of the liver parenchyma, and gradually connected with each other and also with hepatic ducts. the discontinuous laminin immunofluorescence also appeared in the endodermal cells around the portal vein branches at the younger stages. Therefore, it is conceivable that the intrahepatic bile ducts originate from discrete cell populations of type I hepatocytes around the portal vein branches and subsequently become confluent, but not from the cells of hepatic ducts.