Introduction: Increased oxidative stress is involved in the pathology of cardiovascular disease. Transient congenital hypothyroidism (TCH) leads to a variety of heart disorders during adulthood. In this study, the effect of TCH on antioxidant and oxidant systems in the serum and left cardiac ventricle of adult male rats was investigated. Materials and Methods: TCH was induced by administrating 6-propyl-2-thiouracil in the drinking water of pregnant Wistar rats throughout pregnancy, while pregnant rats in the control group consumed only tap water. Superoxide dismutase, catalase (CAT) enzymes, total antioxidant capacity (TAC) and malondialdehyde were measured in the serum and the left ventricles of adult (16 weeks) male offspring. Lipid peroxidation index (LPI) was calculated in serum and cardiac tissue. Results: The results of this study show that CAT activity, TAC level and also LPI in the serum of rats in the TCH group were significantly higher than in controls. In the left ventricular tissue of the TCH group, compared to the control group, CAT activity (107.5±13.1 vs. 159.6±12.3 KU/L, P=0.013) and TAC levels (172.2±7.6 vs. 235.6±13.1 µmol/L, P=0.001) were significantly lower. Conclusions: Decreased CAT activity and TAC in left ventricular cardiac tissue show that TCH can lead to increased oxidative stress of cardiac tissue in offspring. Increased cardiac oxidative stress may therefore contribute to the decreased cardiac function observed in patients with TCH.