BACKGROUND: Equine metabolic syndrome is an endocrinopathy that affects different breeds of horses and ponies. This metabolic disorder is a multifactorial syndrome and it is primarily linked to obesity, dyslipidemia and insulin resistance. OBJECTIVES: The present study was performed to evaluate insulin resistance in Darehshori horses with regards to age, sex, season and obesity. METHODS: Twelve adult stallions and 14 non-pregnant Darehshori breed mares were selected between January and November 2015. Fourteen horses were under and others were above 10 years old. Body condition scores of the animals were estimated based on Hennekes’ 1 to 9 scoring system, thus, 1 is poor and 9 is extremely fat horse. Body condition score of 12 horses was under 5 and the others above 5. The horses were studied during a period of one year, every 45 days and blood sampling and body condition scoring were performed at each time. Insulin resistance was assessed based on screening test. Serum concentrations of glucose, insulin and leptin were determined. RESUTLS: This study comprehensively used multiple detailed parameters to scan thoroughly the concept of insulin resistance in Darehshori horses on a long term basis. The levels of insulin, glucose and leptin in aged and obese stallions in warmer months were significantly higher than the others and reference range (P<0.05), hence, insulin resistance was detected in these groups of studied animals. CONCLUSIONS: These findings represented insulin resistance phenomenon in a specific breed in Iran. The physiologic characteristics of this breed may be different from other ones and the conditions in Iran such as climate are different from other countries. Hence, it should be considered that Darehshori horses have unique characteristics compared with other breeds. The study may be used as a guideline for evaluating the insulin resistance in Darehshori horses.