of inflammation and fibrosis in the pancreatic islets in diabetes is largely unknown. Spontaneously diabetic Torii (SDT) rats exhibit inflammation and fibrosis in and around the islets during the development of the disease. We investigated genetic factors for diabetes, islet inflammation, and fibrosis in the SDT rat. We produced F1 and F2 rats by intercross between SDT and F344 rats, examined the onset of diabetes, glucose tolerance, and histology of the pancreas, and performed genetic analysis of these traits. We then established a congenic strain carrying the SDT allele at the strongest diabetogenic locus on the F344 genetic background and characterized glucose tolerance and histology of the pancreas. F1 rats showed glucose intolerance and inflammatory changes mainly in the islets. Genetic analysis of diabetes identified a major locus on chromosome 3, designated Dmsdt1, at which a dominantly acting SDT allele was involved. Quantitative trait locus (QTL) analysis of glucose tolerance revealed, in addition to Dmsdt1 [logarithm of odds (LOD) 5.3 near D3Mit12], three other loci, designated Dmsdt2 (LOD 4.2 at D8Rat46), Dmsdt3 (LOD 3.8 near D13Arb5), and Dmsdt4 (LOD 5.8 at D14Arb18). Analysis of a congenic strain for Dmsdt1 indicates that the dominantly acting SDT allele induces islet inflammation and fibrosis. Thus we have found a major locus on chromosome 3 for islet inflammation and fibrosis in the SDT rat. Identification of the genes responsible should provide insight into the pathogenesis of diabetes. congenic analysis; diabetes; genetic factor; glucose tolerance; quantitative trait locus DIABETES MELLITUS IS a multifactorial disease involving interaction of genetic and environmental factors. Spontaneous animal models of diabetes are widely used in research and have provided valuable information on the nature of the disease. Inflammation and fibrosis in pancreatic islets have been described in human type 2 diabetes (4) and in spontaneous animal models of type 2 diabetes, including the Goto-Kakizaki (GK) rat (12), the Zucker diabetic fatty rat (33), and the Otsuka Long-Evans Tokushima fatty (OLETF) rat (16). A role of inflammation and fibrosis in the pathogenesis of type 2 diabetes has been suggested (17), but the mechanism and genetic factors remain to be elucidated. Genetic, pathophysiological, and histological analyses using animal models should provide useful information for understanding the molecular mechanism of the inflammatory process in the development of the disease.The spontaneously diabetic Torii (SDT) rat was established from an outbred colony of Sprague-Dawley rats in 1997 as a nonobese diabetic animal model (37,38). Male SDT rats develop diabetes at 15 wk of age or later and show 100% incidence of diabetes by 40 wk of age. Pathological changes including microvascular congestion and hemorrhage in pancreatic islets appear from ϳ8 wk of age and are followed by inflammation and fibrosis in and around the islets. These pathological changes in the pancreas of SDT rats differ from those of typical autoi...