An fa allele of the leptin receptor gene (Lepr^fa) of the Zucker fatty rat was introduced into the genome of the Spontaneously Diabetic Torii (SDT) rat, an inbred model of nonobese type 2 diabetes mellitus, through the `Speed congenic method'. The newly established congenic strain of a SDT rat for Lepr^fa was maintained by intercrossing between fa-heterozygous littermates, and the phenotypes related to obesity and diabetes were investigated till 32 wks of age. SDT fa/fa rats of both sexes exhibited obesity, adiposity and insulin resistance associated with hyperphagia from the loss of leptin action. Interestingly, they developed diabetes from 5 wks of age in males and 8 wks in females with the incidences reaching 100% at 16 wks in males and 73% at 32 wks in females. In contrast, heterozygous (+/fa) and wild-type (+/+) rats developed spontaneous nonobese diabetes in males from approximately 20 wks, but not in females, as with the original SDT rats. These results indicate that the fa gene accelerates the onset of diabetes in SDT rats by making adiposity and/or insulin resistance as potent risk factors for development of their diabetes. The SDT.Lepr^fa congenic rat strain is expected to be a novel model of obesity-related diabetes and could be a useful tool for studies of the genetic backgrounds of diabetes in response to fa-induced obesity.