L-Arginine supplementation has beneficial effects on metabolic disorders in rodents. We here investigated the effects of exogenous L-arginine on cardiac pathology and mitochondrial reactive oxygen species (ROS) production and dynamics in DahlS.Z-Lepr/Lepr (DS/obese) rats, a model of metabolic syndrome (MetS). DS/obese rats and their lean homozygous littermate (DahlS.Z-Lepr/Lepr, or DS/lean) controls were provided with drinking water containing 0.50% L-arginine-HCl or 0.85% L-alanine (isonitrogenous control) from 13 to 17 weeks of age. L-Arginine supplementation markedly alleviated hypertension without affecting cardiac injury in MetS rats. It also attenuated the increase in ROS production apparent in cardiac mitochondria isolated from MetS rats as well as suppressed the associated upregulation of Nox4 mRNA and protein in the heart. Furthermore, L-arginine reversed the decrease in the size of cardiac mitochondria as well as changes in the expression of DRP1 and OPA1 proteins apparent in the L-alanine-treated MetS rat heart. Cardiac arginase II gene expression and arginase activity were increased by L-arginine treatment in MetS rats but not CONT rats. L-Arginine supplementation thus ameliorated hypertension and cardiac mitochondrial abnormalities in MetS rats, with the lack of a cardioprotective effect possibly being due to increased arginase activity.