Blood flow requirements of the respiratory muscles (RM) increase markedly during exercise in chronic heart failure (CHF). We reasoned that if the RM could subtract a fraction of the limited cardiac output (QT) from the peripheral muscles, RM unloading would improve locomotor muscle perfusion. Nine patients with CHF (left ventricle ejection fraction= 26 ± 7 %) undertook constant-work rate tests (80% peak) receiving proportional assisted ventilation (PAV) or sham ventilation. Relative changes (%) in deoxy-hemoglobyn ([HHb]), oxi-Hb ([O<inf>2</inf>Hb]), tissue oxygenation index (TOI) and total Hb ([HbTOT], an index of local blood volume] in the vastus lateralis were measured by near infrared spectroscopy. In addition, QT was monitored by impedance cardiography and arterial O<inf>2</inf> saturation by pulse oximetry (SpO<inf>2</inf>). There were significant improvements in exercise tolerance (Tlim) with PAV. Blood lactate, leg effort/Tlim and dyspnea/Tlim were lower with PAV compared to sham ventilation (P<0.05). There were no significant effects of RM unloading on systemic O<inf>2</inf> delivery as QT and SpO<inf>2</inf> at submaximal exercise and at Tlim did not differ between PAV and sham ventilation (p>0.05). Unloaded breathing, however, was related to enhanced leg muscle oxygenation and local blood volume compared to sham, i.e., higher [O<inf>2</inf>Hb]% and [HbTOT]%, respectively (p<0.05). We conclude that RM unloading had beneficial effects on the oxygenation status and blood volume of the exercising muscles at similar systemic oxygen delivery in patients with advanced CHF. These data suggest that blood flow was redistributed from respiratory to locomotor muscles during unloaded breathing.