We investigated whether endothelium-derived relaxing (EDRF) and hyperpolarizing factor (EDHF) is impaired in type 2 diabetic rats (Otsuka Long-Evans Tokushima Fatty (OLETF) rat) and whether the exercise training improves impaired EDRF and EDHF. Diabetic rats were divided into the sedentary and exercise-trained groups at the age of 16 weeks. Long-Evans Tokushima Otsuka (LETO) rats were used as age-matched non-diabetic controls. EDRF as well as EDHF induced by acetylcholine in the presence of indomethacine and L-nitro N-arginine was significantly attenuated in the diabetic rats, and was further impaired with age. Exercise training significantly improved it. Both insulin resistance and abdominal fat accumulation were significantly greater in the diabetic rats, compared with the non-diabetic rats, but were decreased in exercise-trained rats. Urinary NO(2) secretion was decrease in the diabetic rats at each age, and it was improved by exercise training. The results of the study indicated that exercise training prevented impairment of EDHF, as well as EDRF in type 2 diabetic rats, presumably due to improvement of hyperglycemia and insulin resistance and increase in the production of nitric oxide by exercise training.