Objective: Melatonin is a powerful endogenous antioxidant and it may play a role in prevention of radiation-induced damage. The aim of this study was to investigate the effect of melatonin on bone mineral density in rats receiving radiation. Materials and Methods: Sprague Dawley rats were divided into four groups. Group 1 (control group) received neither melatonin nor radiation (control group). Group 2 (Mel group) was administered intraperitoneal injections of 5mg/kg melatonin daily for ten days. Group 3 (RT group) and Group 4 were exposed to total cranium radiation of 5 Gy in a single dose by using a cobalt-60 teletherapy unit. In addition to irradiation, group 4 (RT + Mel group) was administered 5mg/kg of melatonin intraperitoneally. At the end of the 10th day, the rats' cranium and vertebrae bone mineral densities (BMDs) were measured. Results: When cranial BMDs were evaluated, statistically more significant BMD increases were seen in the Mel group and the RT + Mel groups than in the control group. No significant difference was seen in the Mel group versus the RT + Mel group; however, there was a significant difference between RT and RT + Mel groups. When vertebral BMDs were evaluated, the only significant difference was found between the control and Mel groups. Conclusion: We think that melatonin is a radioprotective agent. However, we would like to emphasize that further studies are needed before clinical trials with melatonin are initiated.