This experimental study was designed to produce ischemia-reperfusion (I/R) injury in rat corpus cavernosum by inducing 1 h of priapism and investigating the effects of melatonin on the levels of oxidative injury parameters. Twenty-one adult male rats were randomly divided into three groups as follows; sham operated, control group (Group C): only penectomy was performed and blood (3 ml) drawn from vena cava inferior (VCI), ischemia and reperfusion group (Group I/R); priapism (1 h) + ½ h reperfusion + penectomy + blood from VCI, melatonin treatment group (Group I/R + M); priapism (1 h) + melatonin (½ h before reperfusion, 50 mg/kg, ip) + ½ h reperfusion + penectomy + blood from VCI. Priapism was induced with a vacuum erection device (cut tip of 2-cc syringe) and a rubber band was placed at the base of the erect penis. In two groups, excluding Group C, penectomies were performed after 1 h of ischemic priapism and ½ h reperfusion for biochemical analysis of superoxide dismutase (SOD), catalase (CAT), malondialdehyde (MDA) and protein carbonyl (PC) in the tissues. In all groups, about 3 ml blood was drawn from VCI to study the same parameters in systemic circulation. The results were compared statistically using one-way analysis of variance (ANOVA). As a result, in biochemical examination of penile tissues, there were significant increase in SOD, CAT activities and MDA levels in I/R group in comparison with group C (P < 0.05). With melatonin treatment, these levels were decreased closer to control levels (P < 0.05). The changes in PC levels were insignificant in penile tissues of all groups (P > 0.05). Analysis of serum in all rats revealed that the activity of SOD and the levels of MDA, NO and PC were increased in I/R group when compared with control group but with multiple comparisons only the increases in SOD activity and NO level were significant (P < 0.05). Decrease in the activity of SOD and the levels of NO and PC were significant after melatonin administration in serum of all groups (P < 0.05). The results of this study showed that experimentally induced priapism caused oxidative injury in cavernosal tissues of rats, and treatment with melatonin alleviated these effects. From the result of this experimental study, it can be extrapolated that melatonin may be used as an antioxidant agent in the treatment of ischemic priapism in the future urology practice.