We have immunohistochemically investigated the occurrence of pi class glutathione S-transferase (GST-pi) both in normal rat liver and in rat liver regenerating after hepatectomy. We made our observations by examining frozen liver sections rather than the paraffin sections used in previous studies. First we examined normal liver tissue. Light microscopic examination revealed weak, discontinuous immunofluorescence along the hepatocellular plasma membranes, indicating the presence of GST-pi. Then, using immunoelectron microscopy, we detected immuno-reaction products along the bile canalicular plasma membranes, indicating the location of the GST-pi more precisely. On the second and third days after hepatectomy, as the liver was regenerating, we observed that both the immunofluorescence and the reaction products were much more intense and that the bile canaliculi were dilated as well. These observations indicate that in regenerating liver, GST-pi which is a cytosolic enzyme, concentrates in the cytosol of the hepatocytes directly under the bile canalicular plasma membranes. These observations are the first reported morphological confirmation of induced GST-pi in regenerating liver.