Twelve untrained aged mares were used to determine whether 7 days of light exercise improved peripheral tissue insulin sensitivity. Mares were divided into obese-exercised (n = 3), obese-sedentary (n = 3), lean-exercised (n = 3) and lean-sedentary (n = 3) groups. The exercised groups were worked at a trot to a heart rate (HR) of not more than 140 beats/min for 30 min in a round pen. Each group was subjected to 3 euglycaemic hyperinsulinaemic clamps: prior to exercise (P), 24 h following the seventh exercise training session (E) and 9 days postexercise training (PE). Prior to exercise training, the mares in the obese group were confirmed insulin-resistant compared to the mares in the lean group. There was no change in bodyweight or body condition in the obese or lean groups throughout the study. Glucose infusion rate (GIR) was higher (P<0.05) on E compared to P days in the obese-exercised and lean-exercised groups. Insulin sensitivity returned to pre-exercise values by 9 days postexercise in the obese-exercised and lean-exercised groups. The results of this study suggest that improvement in insulin sensitivity occurs in obese mares without a long interval of exercise training and in the absence of a change in bodyweight.