Reactive oxygen species are produced under diabetic conditions and possibly cause various forms of tissue damage in patients with diabetes mellitus. The aim of this study was to examine the effects of high glucose on the alloxan-induced beta cell injury. The insulinoma (RINm5F) cells were clutured either with high glucose (22.2 mM) or normoglucose (5.6 mM) in RPMI 1460 media for 3 days. The SOD activities were determined by spectrophotometric assay and nitroblue tetrazolium (NBT) stain. The effects of high glucose on the cytotoxicity of alloxan were also investigated in RINm5F cells and the cells viability were determined by 3-(4,5-dimethylthiazolyl)-2,5-diphenyltetrazolium bromide (MTT) methods. Results showed that the CuZn-SOD activity was decreased but Mn-SOD activity was increased significantly in RINm5F cells cultured with high glucose (22.2 mM) media. The cytotoxicity of alloxan was increased by high glucose compared with normoglucose in RINm5F cells. Diethyl-dithiocarbarmate (DDC), as inhibitor of CuZn-SOC, also potentiate the alloxan-induced cytotoxocity in RINm5F cells. These results suggest that, in RINm5F cells, short term culture with high glucose media decreases Cu-Zn-SOD activity and the decreased activity of CuZn-SOD many one of the causative factors of beta-cell injury induced by high glucose.