The aim of this study was to determine changes in the concentration of leptin and thyroid hormone in energyrestricted ewes and effect of leptin injection on
thyroid hormone level. In the first experiment, 28 ewes were assigned to two groups(n=14). Ewes were fed by a ration that provided 60% or 100% of maintenance energy requirements during 71 days. Blood samples were collected and body weight(BW) and body condition score(BCS) were determined. In the second experiment, 6 energy-restricted ewes from the first experiment were selected and assigned to two groups(n=3). Ewes were fed a ration that provided 60% of maintenance energy requirements and injected by 1 or 4 Kg leptin/Kg BW.Blood samples were taken and BW and BCS were determined. In the first experiment, concentrations of leptin, T3 and T4, BW and BCS were decreased in energyrestricted ewes(p<0.01). In the second experiment,concentration of T3, T4 and leptin were increased by leptin injection(p<0.01). The results suggested leptin injection may restore thyroid hormone secretion to normal levels in energy-restricted ewes.