<b>Background and Objective:</b> Diabetes mellitus is characterized by hyperglycemia caused by disturbances in pancreatic β cells resulting in the formation of reactive oxygen species which in this study was characterized by increased levels of nitric oxide and malondialdehyde in male Wistar rats with diabetes mellitus models. <b>Materials and Methods:</b> Animal modeling of diabetes mellitus using 40 mg kg<sup>1</sup> b.wt. streptozotocin intraperitoneally. The antioxidant activity is based on the ferric reducing antioxidant power (FRAP) method. Determination of nitric oxide and MDA levels using the Griess method and Thiobarbituric Acid Reactive Substances (TBARS) Assay, respectively. The animal models were divided into six treatment groups, normal control (KN), positive control (K<sup>+</sup>) (glibenclamide), negative control (K<sup></sup>) (Na-CMC 0.5%), teak leaf ethanol extract group at a dose of D<sub>1</sub> (100 mg kg<sup>1</sup> b.wt.), D<sub>2</sub> (200 mg kg<sup>1</sup> b.wt.) and D<sub>3</sub> (300 mg kg<sup>1</sup> b.wt.). <b>Results:</b> The antioxidant activity showed that the IC<sub>50</sub> values of vitamin C and the ethanol extract of <i>Tectona grandis</i> leaves were 18.208 and 62.236 μg mL<sup>1</sup>, respectively. The NO levels in KN = 83.133 μmol L<sup>1</sup>, K<sup>+</sup> = 118.300 μmol L<sup>1</sup>, K<sup></sup> = 317.467 μmol L<sup>1</sup>, D<sub>1</sub> = 210.133 μmol L<sup>1</sup>, D<sub>2</sub> = 184.467 μmol L<sup>1</sup> and D<sub>3</sub> = 129.300 μmol L<sup>1</sup>. The MDA levels at KN = 3.767 μmol L<sup>1</sup>, K<sup>+</sup> = 8.854 μmol L<sup>1</sup>, K<sup></sup> = 31.032 μmol L<sup>1</sup>, D<sub>1</sub> = 27.010 μmol L<sup>1</sup>, D<sub>2</sub> = 20.166 μmol L<sup>1</sup> and D<sub>3</sub> = 15.512 μmol L<sup>1</sup>. <b>Conclusion:</b> The ethanol extract of teak leaves <i>Tectona grandis</i> L. at a dose of 300 mg kg<sup>1</sup> reduces nitric oxide levels and plasma malondialdehyde levels in rats through the activity of antioxidant compounds.
Keywords: Antioxidant; Tectona grandis L.; blood glucose; diabetes mellitus (DM); malondialdehyde; nitric oxide.