BACKGROUND: Neogenesis of the beta calls from ductal cells is the main mechanism of the increased beta cell mass after partial pancreatectomy. For the transdifferentiation from the duct cells to the beta cells, de-differentiation of the duct cells is needed because duct cells are also terminally differentiated cells already. But there was no clear evidence of de-differentiation of the duct cells during duct call proliferation so far. Herein we report the changes of intermediate filament protein expression in rapidly proliferating duct cells after partial pancreatectomy for the evidence of de-differentiation of the duct cells. METHODS: 45 week-old Sprague-Dawley rats weighing 80~120 g were used. 90% partial pancreatectomy was done. Experimental animals were divided into 5 subgroups by date of killing after surgery: 1, 3, 7, 14, 30 days, Pancreas remnant was excised and immunohistochemical stain was done for pancytokeratin (Pan-CK) as a epithelial cell marker and vimentin (VT) as a mesenchymal cell marker. We observed the double stained slide with pan-CK and VT antibody using confocal microscope for costaining analysis over time. The sections were also immunostained with anti-insulin antibody for the quantification of the beta cell mass by point-counting methods. RESULTS: We observed impaired glucose tolerance and diabetes were developed affer 90% pancreatectomy. Significant increase of the weight of pancreatic remnant, beta cell and duct cell mass were observed about 14 days after pancreatectomy. We observed the co-expression of VT and pan-CK intermediate filament protein in rapidly proliferating duct cells in the area of common pancreatic duct and main duct at one day after partial pancreatectomy. 3 days affer partial pancreatectomy, VT and pan-CK costained duct cells were mainly observed in the rageneration focus of the duct cell proliferation. 30 days after partial pancreatectomy, we could not find any costaining duct calls in the remnant pancreas. CONCLUSION: The vimentin intermediate filament, a marker of mesenchymal cell was expressed in proliferating ductal cells after pancreatectomy. We could suspect that pancytokeratin and vimentin co-expression is a good marker for de-differentiation of proliferating duct cells.