Sympathetic nervous system restraint of skeletal muscle blood flow during dynamic exercise has been well documented. However, whether sympathetic restraint of muscle blood flow persists and is constant throughout prolonged exercise has not been established. We hypothesized that both ₁ - and ₂ - adrenergic - receptors would restrain skeletal muscle blood flow throughout prolonged constant - load exercise, and that the restraint would increase as a function of exercise duration. Mongrel dogs were instrumented chronically with transit-time flow probes on the external iliac arteries and an indwelling catheter in a branch of the femoral artery. Flow-adjusted doses of selective ₁ - (prazosin) and ₂ - adrenergic - receptor (rauwolscine) antagonists were infused following 5, 30 and 50 min of treadmill exercise at 3 and 6 miles/h. During mild-intensity exercise (3 miles/h) prazosin infusion resulted in a greater (p<0.05) increase in vascular conductance (VC) following 5 (42 ± 6 %), compared to 30 (28 ± 6 %) and 50 (28 ± 8 %) min of running. In contrast, prazosin resulted in a similar increase in VC following 5 (29 ± 10 %), 30 (24 ± 9 %) and 50 (22 ± 9 %) min of moderate-intensity (6 miles/h) exercise. Rauwolscine infusion resulted in a greater (p<0.05) increase in VC following 5 (39 ± 14 %), compared to 30 (26 ± 9 %) and 50 (22 ± 4 %) min of exercise at 3 miles/h. Rauwolscine infusion produced a similar increase in VC following 5 (19 ± 3 %), 30 (15 ± 6 %) and 50 (16 ± 2 %) min of exercise at 6 miles/h. These results suggest that the ability of ₁ - and ₂ - adrenergic - receptors to produce vasoconstriction and restrain blood flow to active muscles may be influenced by both the intensity and duration of exercise.