The obesity of the Zucker rat causes oxidative stress in different cellular structures, as observed by the increase in lipid peroxidation markers (TBARS and urinary 8-isoprostane), protein (3-NT) and mitochondrial DNA (urinary 8-OHdG) oxidation, and dysregulation in antioxidant enzymes more present in the cytoplasm (CAT), in the mitochondria (SOD), and in the general antioxidant balance (GSH/GSSG ratio and TE-Trolox equivalent) (CTL-Obese, Obese+AT, and Obese+AT+L-NAME compared to CTL-Lean; p < 0.0001). The gene discussed is CAT; the disease is Obesity.