At any intensity of exercise, the attainable oxygen consumption (V˙O2) is a product of the interaction between perfusive (Q˙O2) and diffusive (DO2) transport of O2. The exercise intolerance of chronic heart failure (CHF) that increases morbidity and mortality results from impaired Q˙O2 and DO2. The key determinant of DO2 is red blood cell (RBC) volume contained within flowing capillaries. The proportion of capillaries supporting RBC flux (%cap) is substantially reduced in CHF which may be due to decreased nitric oxide (NO) bioavailability. Nitrate (NO3-) supplementation via beetroot juice (BR) increases the concentrations of NO precursors (NO3-, nitrate) in the blood and also increases V˙O2max in CHF patients. PURPOSE: We hypothesized that by increasing the proportion of capillaries supporting RBC flux, BR would increase DO2 at rest and during contractions in CHF rats. METHODS: CHF was induced in young adult male Sprague-Dawley rats by surgically induced myocardial infarction (MI). Following a 5-week recovery period rats were given BR ([NO3-] 1 mmol/kg/day, CHF+BR) or placebo (CHF) for 5 days. MI size was not different between groups (CHF 28 ± 5, CHF+BR 28 ± 6 %). Intravital miscroscopy was used to study the in vivo spinotrapezius muscle circulation at rest and during twitch contractions (180 s, 1 Hz, 6 – 8 V). RESULTS: BR increased %cap at rest (76 ± 2 vs 65 ± 6 %, P < 0.05) and during contractions (82 ± 2 vs 78 ± 6 %, P < 0.05) in CHF+BR vs. CHF. DO2 was increased 17% at rest and 5% during contractions in CHF+BR. CONCLUSION: These findings support that 5 days of BR supplementation is an effective means for increasing DO2 in CHF rats and this, in concert with elevated Q˙O2, will enhance O2 transport at rest and during exercise. In addition, because V˙O2 kinetics is O2-delivery dependent in CHF patients, enhanced perfusive and diffusive O2 transport will likely speed V˙O2 kinetics and reduce the O2 deficit incurred during metabolic transition.