Objective: Anabolic-androgenic steroids (AAS) are nominated for clinical use to promote protein synthesis in many therapeutic conditions. However, the indiscriminate use of AAS is related to hazardous cardiac disturbances and oxidative stress. We designed a study to investigate whether prolonged treatment with high doses of stanozolol modifies the activities of some antioxidant enzymes in the heart in sedentary and trained rats and whether this treatment causes alterations of cardiovascular parameters. In addition, the effectiveness of melatonin as an antioxidant and as a modulator of the cardiovascular side effects of stanozolol (STA) treatment was analyzed. Materials and Methods: Thirty male Wistar rats were divided into the following six groups: sedentary (S), stanozolol sedentary (SS), stanozolol-melatonin sedentary (SMS), trained (T), stanozolol trained (ST) and stanozolol-melatonin trained (SMT). The stanozolol-treatment rats received 5 mg.kg-1 by subcutaneous injection before each exercise session (5 d.wk-1, i.e., 25 mg.kg-1.wk-1), while control groups received only saline solution injection. The melatonin-treatment groups received intraperitoneal injections of melatonin (10 mg.kg-1), 5 d.wk-1 for 6 wk. Electrocardiography, blood pressure and antioxidant enzyme activity measurements were performed at the end of the experimental period for cardiac function and molecular assessment. Results: This is the first time that the in vivo effects of melatonin treatment on stanozolol-induced cardiovascular side effects have been studied. Stanozolol induced bradycardia and significantly increased cardiac superoxide dismutase and catalase activities. Trained stanozolol-treated rats experienced an increase in blood pressure and relative heart weight, and they developed left cardiac axis deviation. 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