Circulating levels of the potent vasoconstrictor peptide endothelin-1 (ET-1) are increased in congestive heart failure (CHF). Coronary blood flow and myocardial oxygen consumption (MVO2) are decreased in some models of CHF. This study tested the hypothesis that ET-1 induced coronary vasoconstriction limits oxygen availability in the failing heart. The effects of selective ET-A receptor blockade with BQ610 (5 μg/min, intracoronary) and selective ET-B receptor blockade with BQ788 (5 μg/min, intracoronary) on coronary blood flow were examined at rest and during graded treadmill exercise in 8 dogs in which congestive heart failure (CHF) had been produced by rapid ventricular pacing for three to four weeks. In animals with CHF, ET-B receptor blockade caused no change in left ventricular (LV) pressure or coronary blood flow. In contrast, ET-A blockade with BQ610 resulted in modest significant increases of coronary blood flow at rest (from 22.4 ± 2.1 to 27.9 ± 3.0 mL/min) and during two exercise stages (from 26.9 ± 2.0 to 30.7 ± 1.9 during stage 1 exercise and from 28.5 ± 2.0 to 31.7 ± 1.3 mL/min during stage 2; all P < 0.05), with an upward shift in the relationship between coronary flow and rate-pressure product. The increase in coronary flow produced by ET-A blockade was not associated with an increase of either myocardial oxygen uptake or LV dP/dt. Thus, although ET-A receptor blockade caused a modest increase in coronary flow, this did not result in an increase of MVO2, implying that ET-A-mediated coronary vasoconstriction did not limit oxygen uptake by the failing heart.