A variety of mechanisms have been identified as responsible for the onset of diabetes mellitus. Alloxan shows a selective cytotoxicity on pancreatic b-cells and thus causes insulin-dependent (type I) diabetes mellitus. Although the mechanism of alloxan cytotoxicity is not yet clearly understood, several researchers 1,2) demonstrated that the diabetogenic action was initiated by the generation of reactive oxygen species (ROS). Alloxan is a mild oxidant and is easily reduced to alloxan radicals (A· Ϫ ) by GSH or ascorbic acid. 3) Our previous study 4) demonstrated that A· Ϫ can directly reduce O 2 and ferric ions to superoxide anion radical (O 2 · Ϫ ) and ferrous ions, respectively. The extreme and indiscriminate reactivity of ROS could easily explain the killing of b-cells by alloxan. In support of this hypothesis alloxan diabetes is prevented by HO · scavengers, 5-7) superoxide dismutase (SOD) 2) and vitamin E. 8) In addition, on the basis of in vivo and in vitro studies using rats and isolated islet cells, respectively, it was proposed that the process of alloxan toxicity involved the generation of HO · by which the DNA strands break of pancreatic islets is attacked to produce. 9-11) These findings indicate that DNA strand breaks are involved in the events of alloxan-diabetogenesis. However, the complex sequence of events that eventually leads to the DNA strand breaks and death of pancreatic b-cells in alloxan diabetes mellitus is not clear.Cell death might occur by one of two fundamental processes, necrosis or apoptosis. The biochemical hallmarks of apoptosis are the distribution of phosphatidylserine (PS) at the external surface of the plasma membrane (PS exposure) in the early stages, and the cleavage of chromosomal DNA into nucleosomal units, which appears to be the final blow in the cell death process. Recent studies have shown that different trigger such as IL-1b 12) and streptozotocin 13) can induce apoptosis in pancreatic b-cells. Although detailed studies indicate that alloxan can induce insulin-dependent diabetes mellitus, nothing is known about apoptotic action or the sequence of cellular events of alloxan on the b-cells.According to current understanding, mitochondrial damage seems to occur in the early phase of apoptosis and to participate in the control of apoptosis. 14,15) Release of cytochrome c and apoptosis induced factor from mitochondria into cytoplasm during mitochondrial permeability transition cause to active caspase family, leading to the completion of apoptosis. 15,16) Isolated liver mitochondria incubated with alloxan alters respiration, decreases in the concentration of adenine nucleotides, causes Ca 2ϩ release, and changes in the Dy, 17,18) indicating that alloxan directly induces the mitochondrial damage. However, the relationship between diabetes, apoptosis and mitochondrial damage has remained elusive.INS-1 cells were established by X-ray-induced rat transplantable insulinoma, and have retained the similar ability of glucose-stimulated insulin release of native b-cells. 19) INS-1 ...