Progressive loss of pancreatic b-cell mass is a crucial feature of type 2 diabetes mellitus. As b-cells express very low amounts of the antioxidant enzymes catalase and glutathione peroxidase (GPx), they appear to be particularly vulnerable to oxidative damage in the pathogenesis of diabetes. Here, we investigated the pancreatic expression pattern and regulation of selenoprotein P (Sepp1), which may serve as an additional antioxidant enzyme inside and outside of cells. Sepp1 was detected in rodent pancreas by immunofluorescence and real-time RT-PCR. Regulation of Sepp1 biosynthesis in INS-1 rat insulinoma cells was investigated by real-time RT-PCR, luciferase gene reporter assay, and immunoblotting. Sepp1 and Gpx1 gene expressions in rat pancreas were 58 and 22% respectively of the liver values. Pancreatic Sepp1 expression was restricted to the endocrine tissue, with Sepp1 being present in the a-and b-cells of mouse islets. In INS-1 insulinoma cells, Sepp1 expression was stimulated by the selenium compound sodium selenate and diminished in the presence of high glucose (16 . 7 vs 5 mM) concentrations. Sepp1 mRNA stability was also lowered at 16 . 7 mM glucose.Moreover, Sepp1 mRNA levels were decreased in isolated murine islets cultured in highglucose (22 mM) medium compared with normal glucose (5 . 5 mM) medium. Pancreatic Sepp1 expression was elevated upon treatment of mice with the b-cell toxin streptozotocin. This study shows that pancreatic islets express relatively high levels of Sepp1 that may fulfill a function in antioxidant protection of b-cells. Downregulation of Sepp1 expression by high glucose might thus contribute to glucotoxicity in b-cells. Key Words" pancreatic islet