Abstract: Background: Boldenone is developed for veterinary use and recently used by bodybuilders in both off-season and pre-contest where it is well known for increasing vascularity while preparing for a bodybuilding contest. So, the present study was designed to investigate the possible effect of growth promoter boldenone undecylenate on total antioxidant capacity and oxidative stress on muscular tissues (skeletal, cardiac and smooth muscles). Methods: Forty adult New Zealand rabbits were divided into four groups (10 animals each). Control group includes animals that injected intramuscularly with olive oil and dissected after 3 weeks. The experimental groups include animals that receive one, two and three intramuscular injections of 5 mg kg-1 body weight boldenone, respectively. The animals were dissected after 3, 6 and 9 weeks, respectively where the interval of each dose of boldenone was three weeks. Results: The animals from practice appeared healthy and did not show clinical signs of disease and no rabbit was dead during experiment. Total antioxidant capacity represented in superoxide dismutase (SOD) and reduced glutathione (GSH) in different muscle tissues showed a significant increased in the boldenone treated groups compared with their value in control group. And the oxidative stress parameters such as malondialdehyde (MDA), nitric oxide (NO), total lipid and total protein values showed a significant increased in the boldenone treated groups compared with their value in control group. These alternations were increased with the increase the boldenone dose injection from one dose to three doses, respectively. Conclusion: These findings explain the common phenomena in athletics and bodybuilders who suffer from muscles damage (rhabdomyolysis), atherosclerotic heart disease and myocardial hypertrophy as they injected with some drugs as steroids (boldenone) to build muscles.