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Background: Heat stress reduces reproductive performance in farm animals. The aim of this study was to determine the effect of heat stress and different concentrations of melatonin on nuclear maturation of ovine oocytes. Materials and Methods: In this experimental study, ovary collection and oocyte recovery were carried out by standard methods. Oocytes culture was in A: TCM199+10% FBS, 5µg/ml FSH, 0.01IU/ml LH, 100 IU/ml penicillin, and 100 IU/ml streptomycin, B: A+heat stress at 40 C0, and C and D:B+1 and 10 µM melatonin, respectively. Results: Heat stress significantly (P<0.05) decreased nuclear maturation in the treatment group in comparison with the control group (60.60 vs. 84.89). Also, 1 and 10 µM melatonin could improve oocytes to reach metaphase-II stage (60.60 vs. 76.92, 78.82, respectively). However, increasing the melatonin dose from 1 to 10 µM did not alter oocytes maturation. Conclusion: Overall, this study showed that melatonin improves ovine immature oocytes maturation during heat stress.