Protective Effect of Melatonine Against Radiation Induced Nephrotoxicity in Rats 13, 4101-4105 IntroductionKidneys are frequently included into the therapy area during radiotherapy applications. The extent of radiation damage depends on both dose and volume of radiation (Perez et al., 2008). Renal tolerance dose is 20 Gy in bilateral irradiation, while in unilateral exposure renal dysfunction starts in 15 Gy and renal function is completely lost in doses of 25-30 Gy. Concomitant use of cisplatin some chemicals such as BCNU and actinomycin further lowers the tolerance dose. When renal tolerance dose is exceeded after a latent period of 6 mounts acute nephritis develops, after 18 mounts on chronic nephritis and hiperreninemic hypertension is observed. In histopathologic examination interstial edema secondary to increased capillary permeability is evident in acute phase, (Zaki et al., 2003).Rapid multiplication of the tumoral cell leaves hypoxia and necrotic areas at the center of the tumors. Since radiation is ineffective at these regions, higher radiation doses are usually required at the tumor center to control tumoral multiplication. The normal tissue surrounding the tumor is rich in blood vessels and well oxygenated, AbstractPurpose: The degree of radiation injury to kidneys which are located within the limits of radiotherapy area is determined by the volume and the dose of radiation to which the organ is exposed. When the tolerance dose of the kidney is exceeded after a latent period of 6 months acute nephritis develops and after 18 months chronic nephritis ensues. Melatonin is known to prevent the oxidative injury of toxins and radiotherapy with its free radical scavenging capacity. Methods and materials: In this study 8 weeks old 24 Sprague -Dawley rats were allocated into 4 groups: Control group; Radiotherapy group (20 Gy bilaterally in 5 fractions); Melatonin group (10 mg/kg intraperitoneally), and Melatonin+radiotherapy group (20 Gy Radiotherapy in 5 fractions+ melatonin 10 mg/kg intraperitoneally). After a follow-up period of 6 months BUN was determined in all groups. After rats were euthanized the kidneys were removed for histopathological examination under both light and electron microscopes. Results: After 6 months follow-up, both at light and electron microscopy levels, the radiotherapy group (p<0.05). Conclusion: It was shown in this experimental model that melatonin has protective effects against radiation injury to kidneys.