The fa-gene was transferred from the Zucker rat (13 M strain) to the Wistar Kyoto (WKY) rat. The survey, performed at the 10th generation of backcrossing, showed that Wistar fatty rats (fa/fa), a congenic strain of WKY, developed obesity and obesity-related features, such as hyperinsulinemia and hyperlipemia, in the same manner as Zucker fatty rats. Males, but not females, showed hyperglycemia, glucosuria, and polyuria as early as 8 wk of age. Tolerance and insulin response to oral glucose were decreased with advancing age in males. The diabetic changes appeared to be caused by an interaction between predisposition to develop diabetes in the WKY rat and fa-induced obesity. This is because WKY rats were found to be less sensitive to insulin than Zucker rats by both the glucose tolerance test and the steady-state blood glucose method which estimates overall insulin sensitivity.