BACKGROUND The phenotype of isolated rat hepatocytes changes during in vitro cultivation, but it is not clear whether this process turns back when the cells are returned to an in vivo milieu. Glutathione S-transferase placental form (GST-P), which is not expressed in hepatocytes in vivo, is induced within a few days of cultivation, whereas cytochrome P-450 constitutionally expressed in hepatocytes in vivo rapidly diminishes. We explored the dynamic nature of hepatocytic phenotypes using these two markers and intrahepatic and intrasplenic transplantation of cultured hepatocytes. METHODS Hepatocytes of F344 rats were isolated and cultured for 5 days to form spheroidal aggregates and then implanted into the livers and spleens of congenic analbuminemic rats. Expression of GST-P and P-450 was then examined immunocytochemically. In the intrahepatic case, the implanted hepatocytes could be distinguished from surrounding host hepatocytes by albumin immunostaining. RESULTS The intrahepatically implanted hepatocytes turned GST-P-negative within 5 to 10 days, and re-expressed P-450 to a comparable level to that in surrounding host hepatocytes after 2 days. On the other hand, the hepatocytes implanted within spleens continued to express GST-P for 10 days, and started to express P-450 at extraordinarily high levels after 5 to 10 days. CONCLUSIONS The phenotype of hepatocytes under in vivo and in vitro conditions has proven to be changeable from one to another.