The aim of this study was to determine the relationship between plasma leptin and FSH concentration in Iranian sheep. Forty female Mehraban and Sanjabi sheep were used. All ewes were cyclic and synchronized with cloprestenol. The ewes were divided into two breed groups: Mehraban breed (n = 20) and Sanjabi breed (n = 20), feeding at maintenance level. On the first and second days of estrus cycle, blood samples were collected from the jugular vein. Ovulation number was determined by endoscopy 7 days after the second injection. Mean Plasma leptin concentrations on second day (4.74 +/- 0.15 and 4.68 +/- 0.10 ng mL(-1)) were significantly higher than those on first day (2.64 +/- 0.11 and 2.56 +/- 0.04 ng mL(-1)) for Mehraban and Sanjabi sheep, respectively (p<0.01). Mean plasma FSH concentrations on second day (2.75 +/- 0.17 and 2.74 +/- 0.15 ng mL(-1)) were also significantly greater than those on first day (1.19 +/- 0.05 and 1.19 +/- 0.04 ng mL(-1)) for Mehraban and Sanjabi ewes, respectively (p<0.01). In the present study, positive relationship has been shown between plasma Leptin and FSH concentrations (p<0.01) in Mehraban and Sanjabi sheep. Ovulation rate had a significant difference between Mehraban (1.20 +/- 0.33) and Sanjabi (1.07 +/- 0.1) ewes. Significant differences were not observed between concentrations of FSH and leptin with ovulation rate in both breeds (p < or = 0.01).