The imbalance between prooxidants and antioxidants defenses is known as oxidative stress and is a common phenomenon associated with inflammatory processes of many diseases, including the Chagas disease, a pathology characterized by chronic inflammation similar to autoimmune pathologies. In Brazil, the patients with this pathology in acute phase are treated with benznidazole (BZN) for 30 to 60 days. The present work measured some components of antioxidant systems present in the blood of healthy subjects and patients with chronic chagasic cardiopathy. The levels of the thiobarbituric acid reactive substances (TBARS) and reduced glutathione (GSH) were analyzed in blood (plasma and acid extract, respectively) in three groups: patients treated with BZN (5 mg/kg/day, v.o, 60 days), patients not treated with BZN and healthy subjects (controls). One section was homogenized in phosphate buffer and DTNB to determinate GSH level. Another section was homogenized in TBA solution to determinate TBARS level. The results showed that TBARS levels were significantly increased (p<0.05) in chagasic patients treated with BZN (48.80 ± 5.28 nmol g^-1) when compared with healthy subjects (36.34 ± 6.03 nmol g^-1) and chagasic patients not treated with BZN (32.603 ±  6.94 nmol g^-1). Curiously, there was not significant difference in GSH levels with patients treated (1.67 ±   0.79 mmol) or not treated with BZN (1.64 ±  0.57 mmol) but a significant difference (p<0.05) was detected in relation to healthy subjects (0.82 ±   0.59 mmol). The data obtained suggest that BZN could induce oxidative stress in chronic chagasic patients and that Chagas´ disease also caused oxidative stress in chronic patients.