Objective: Diabetic kidney disease (DKD) has been the most common cause of end-stage renal disease (ESRD) which leads to hemodialysis for more than 20 years in Japan. Various reports have been published on the mechanisms underlying the progression of DKD, however, many details remain unclear. We investigated the mechanisms underlying the progression of renal fibrosis and kidney injury in hyperglycemia from the viewpoint of metabolic alteration in the kidney by using streptozotocin-treated mice (STZ mice). Design/Methods: 8 weeks old ICR mice were treated with streptozotocin (200 mg/kg intra-peritoneally), to produce type 1 diabetes mellitus model mice. The kidneys of those mice were collected 4 weeks after STZ treatment and examined for metabolites that were altered in the kidneys of STZ mice by metabolome analysis. Amino acids that were increasing in the kidneys of STZ mice were administered to L929 murine fibroblast cells, and expression of pro-fibrotic genes were examined. Results: STZ-treatment caused progression of renal fibrosis in mice under both low- and high-protein diets. Metabolome analysis of the kidneys in STZ mice showed marked alterations of amino acids in the kidneys. Increased levels of branched chain amino acids (BCAAs) such as valine and leucine were observed. Among these amino acids, leucine administration to L929 cells resulted in increased expression of genes promoting renal fibrosis. Conclusions: Alterations of BCAAs in the kidneys due to hyperglycemia in STZ mice may affect progression of renal fibrosis and kidney injury. We are now investigating the mechanism by which BCAAs contribute to renal fibrosis.