Influences of genetic and nutritional factors on body weight, fat mass, and leptin production and effects of leptin were assessed in normal [Wistar-Kyoto (WKY)] and diabetic [GotoKakizaki (GK)] rats by mechanism-based modeling. The study included 60 WKY and 60 GK rats; half received high-fat diet (HF), and the others received normal rat chow (N). Body weights and food consumption were measured twice weekly. Six rats per group were sacrificed at 4, 8, 12, 16, and 20 weeks. Abdominal fat was weighed, and plasma leptin was measured by enzyme-linked immunosorbent assay. All data were comodeled using NONMEM version VI level 1.1 (first-order conditional estimation with interaction) (Beal SL, Boeckmann AJ, Sheiner LB, and NONMEM Project Group, NONMEM Users Guides, University of California, San Francisco, CA, 2007). Weight gain was modeled as differences between energy intake and metabolic rate based on allometrically scaled lean body mass (LBM).The GK had higher metabolic rates (1.15 kcal/day/g LBM 0.75 ) than WKY-N (0.92) and WKY-HF (1.02) rats and higher efficiency in transforming energy into body weight. Leptin effect was modeled as inhibition of food consumption. Total body fat was estimated from abdominal fat. Leptin production from fat was 4.7-fold higher for GK (3.03 ng/ml/day/g) than WKY (0.66 ng/ml/day/g). Leptin production rate from LBM was 0.53 ng/ ml/day/g for all groups. The IC 50 for inhibition of food intake by leptin was approximately 3-fold higher in GK versus WKY, indicating leptin resistance for the effect on food consumption in GK. The GK had similar intake of kilocalories but lower body weights and fat mass than WKY, possibly because of higher metabolic rates. Our mechanism-based model explains intrinsic reasons for differences in growth, food intake, and leptin concentrations among these two strains of rats.Type 2 diabetes (T2DM) is a major health risk in many countries and the number of diabetic patients and associated costs for health systems are increasing. More than 180 million people are diabetic worldwide, and this number was estimated to more than double by 2030 (Centers for Disease Control and Prevention, http://www.cdc.gov/diabetes/pubs/ factsheets.htm). In 2005, an estimated 1.1 million people died from diabetes, and such deaths were predicted to increase by more than 50% over the next 10 years (Centers for Disease Control and Prevention, http://www.cdc.gov/ diabetes/pubs/factsheets.htm).