Objective: Glucocorticoids have been used in the treatment of a number of diseases due to their widely therapeutic effects. But they have serious side effects. Although glucocorticoid therapy is an effective measure to prevent and treat many diseases in patients, it can cause adverse effects on the cardiovascular system secondary to oxidative stress. It has been suggested that melatonin is a potent antioxidant and that melatonin supplements may protect against such age-related diseases as atherosclerosis, cancer, and Alzheimer disease. In this study, we investigated whether such effects of a single high dose steroid has adverse consequences for the oxidant-antioxidant system of rabbit heart tissue and whether melatonin treatment is protective in the rabbit. For this purpose we measurument some biochemical parameters in the rabbit heart tissue.