We investigated whether the increase in hindlimb blood flow and vascular conductance in conscious dogs during graded dynamic exercise is functionally restrained by the sympathetic nervous system. Dogs were chronically instrumented to monitor terminal aortic blood flow (TAQ) as an index of hindlimb skeletal muscle blood flow and mean arterial pressure (MAP). The extent of functional sympathetic tone was assessed by measuring the increase in TAQ and terminal aortic vascular conductance (TAC, calculated as TAQ/MAP) in response to intra-arterial infusion of the alpha-adrenergic antagonist prazosin (PZ; 50 micrograms/kg) into the hind-limbs at rest and during steady-state dynamic (treadmill) exercise ranging from mild (3.2 km/h, 0% grade to moderately heavy (8 km/h, 15% grade) workloads. This dose of PZ completely abolished the large hindlimb vasoconstrictor response to phenylephrine (1 microgram/kg ia). At rest, PZ increased TAQ by 0.10 +/- 0.02 l/min and TAC by 1.85 +/- 0.53 ml.min-1.mmHg-1. During exercise, as workload increased and the control levels of TAQ and TAC rose progressively delta TAQ and delta TAC with PZ infusion also increased. At the highest workload, PZ increased TAQ by 0.41 +/- 0.07 l/min and TAC by 4.81 +/- 0.38 ml.min-1.mmHg-1. The increase in TAQ and TAC with PZ were linearly related to the control level of TAQ, indicating that as workload increases progressively greater restraint of muscle vasodilation by the sympathetic nervous system occurs. We conclude that during dynamic exercise in conscious dogs the sympathetic nervous system progressively restrains the normal vasodilation in active skeletal muscle, thereby limiting skeletal muscle perfusion.