Near the center of the brain, there is a small gland named Pineal which produces and releases melatonin during night; which in turn affects hypothalamus and with the help of GnRH, stimulates pituitary gland. Anterior pituitary then releases gonadotropin hormones (LH, FSH), which stimulate and trigger the development of follicles and subsequently ovulation. Therefore, melatonin has a vital role in starting and general performance of reproductive processes. In order to assess the effect of Regulin (melatonin) on reproductive performance of Iranian Afshari sheep breed inside and outside the breeding season, 1590 ewes were evaluated in 6 groups. All ewes were 4 to 5.5 years old and had 2 to 4 times previous breeding. For each 30 ewes, there was one ram in the flock. The experimental treatments were included 1- 18 milligrams of regulin outside breeding season, 2- zero milligram of regulin outside breeding season, 3- 18 milligrams of regulin inside breeding season, 4- zero milligram of regulin inside breeding season. This study was done using a completely random design analysis and the data was analyzed using 2x2 factorial method. Breeding percentage and the rate of twin bearing in the group which received regulin outside breeding season had a significant difference with the control group (without regulin). Moreover, fertility percentage, reduction of infertile ewes, and reduction of breeding period in the group with regulin outside the breeding season had a significant difference to the group without regulin inside breeding season (p<0.05). Abortion percentage on the other hand did not have a significant difference between regulin and control groups. Using regulin has a considerable effect on the reproductive performance of Afshari sheep breed outside the breeding season and given the findings of this study is highly recommended in sheep flocks.

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