The oxygen cost of breathing and blood flow requirements of the respiratory muscles during exercise are discussed along with the implications for limitation of locomotor muscle and exercise performance. Findings show that the oxygen cost of the hyperpnea achieved during very heavy exercise may approach 15% or more of VO2max under conditions that require extraordinary levels of ventilatory work. These conditions include those in the highly trained endurance athlete (at VE > 150 l.min-1), the older athlete at VE of 110-120 l.min-1), and athletic cursorial mammals at VO2max--all of whom experience significant expiratory flow limitation and sometimes even complete ventilatory limitation during heavy or maximum exercise. Rates of blood flow to the respiratory muscles under these peak exercise conditions may equal or exceed those to the limb locomotor muscles. The hypothesis is advanced that excessive requirements of ventilatory work (and therefore VO2 and blood flow) during heavy exercise may cause reflex vasoconstriction of locomotor muscles resulting in curtailment of endurance exercise performance.