Background: Ileal interposition (IT), in which the distal ileum is transposed isoperistaltically into the proximal jejunum, is considered as a procedure for metabolic or antidiabetes surgery. Our aim was to study the effects of IT on glycemic control, fat metabolism, and hormonal changes in obese rats with spontaneous diabetes. Methods: Animals were divided into either an IT or a sham (SH) group. They underwent an oral glucose tolerance test (OGTT) before and 4 and 8 weeks after the operation. All animals were killed 10 weeks after operation for analyses of tissue weight (liver, pancreas, epididymal fat, brown fat), immunoblotting of uncoupling protein-1 (UCP1) protein in brown adipose tissue (BAT), and fasting plasma levels of glucose, insulin, glucagon-like peptide (GLP)-1, peptide YY (PYY), glucose-dependent insulinotropic polypeptide (GIP), and leptin. Results: Body weight increased postoperatively in both groups compared with preoperative weight, but it did not differ between the 2 groups. Eight weeks postoperatively, integrated blood glucose levels during the OGTT were decreased in IT compared with SH (P <.05). Fasting plasma levels of insulin, GLP-1, and GIP did not differ between the 2 groups, but PYY levels were higher in the IT animals (P <.01). The weight of epididymal and BATs, homeostasis model assessment insulin resistance, and fasting plasma leptin levels were decreased in the IT group (P <.05). Expression of UCP1 was higher in IT than SH animals (P <.05). Conclusion: These results suggest that IT improves glucose and lipid metabolism by decreasing insulin resistance and epididymal fat, and increased expression of UCP1 in BAT might be among the mechanisms responsible.