Abstract: Methods for treating diseases in which patients exhibit a reduced exercise capacity due to a decrease in their peripheral blood flow are provided, comprising administering an effective amount of tetrahydrobiopterin to the patient. The patient's resting blood flow is not affected by the administration, and smooth muscle dilating drugs may be administered to the patient concurrently with the tetrahydrobiopterin. Such diseases include chronic heart failure, peripheral vascular disease, diabetes and systemic sclerosis.