Our laboratory has previously reported a decline in sympathetic nervous system restraint of skeletal muscle blood flow during prolonged mild-intensity exercise. This decline may be explained by a decrease in adrenergic receptor responsiveness over time. Thus, the purpose of the present study was to investigate the effect of exercise duration on ₁- and ₂-adrenergic receptor responsiveness during prolonged constant-load exercise. Mongrel dogs (n=6) were instrumented chronically with transit-time flow probes on the external iliac arteries and an indwelling catheter in a branch of the femoral artery. On separate days, flow-adjusted doses of selective ₁- (phenylephrine) ₂-adrenergic receptor (clonidine) agonists and tyramine (to evoke endogenous norepinephrine release) were infused following 5, 30 and 50 min of mild-intensity treadmill exercise (3 miles/h), with hindlimb blood flow (HBF) and mean arterial pressure (MAP) monitored continuously. During treadmill running at 3 mph, infusion of phenylephrine resulted in similar decreases in VC following 5 (73 % (SD 10)), 30 (76 % (SD 9)) and 50 (73 % (SD 10)) min of exercise. Infusion of the ₂-agonist clonidine also produced similar decreases in VC following 5 (58 % (SD 10)), 30 (58 % (SD 11)) and 50 (53 % (SD 12)) min of exercise. Tyramine produced similar decreases in VC following 5 (55 % (SD 15)), 30 (51 % (SD 10)) and 50 (50 % (SD 7)) min of exercise. These results demonstrate that ₁- and ₂- adrenergic receptor responsiveness does not decline during prolonged mild-intensity exercise. Thus, a decrease in - adrenergic receptor responsiveness over time does not appear to be responsible for the decrease in sympathetic restraint of muscle blood flow during prolonged exercise.