OBJECTIVE Whether the dimensions of conduit arteries contribute to the time course of change in blood flow during voluntary rhythmic exercise, and the mechanisms governing such a response in humans, are not known. METHODS The time course of change in the vascular and blood flow dynamics in the brachial artery during the transition between rest and 5 min of rhythmic handgrip exercise was assessed in humans using continuous measures of brachial artery mean blood velocity (MBV; pulsed Doppler), diameter (echo Doppler) and mean arterial pressure (Finapres). The exercise cadence was 1s/1s (Fast) and 1s/2s (Slow) work/rest schedules while supine with the arm positioned above or below the heart. RESULTS Brachial artery diameter of the active arm was reduced 5% at approximately 10 s following the onset of exercise performed above the heart (P < 0.05), irrespective of work rate, and returned to rest levels by 30 s with no concurrent changes in arterial pressure. By 2 min of the Fast contraction rate exercise, brachial artery diameter of the active arm was greater than rest (P < 0.05) irrespective of arm position. Brachial artery dimensions in the contralateral inactive arm were not altered during exercise (P > 0.05). Compared with rest, MBV and forearm blood flow at 5 s of exercise were increased in the active arm but were reduced transiently in the inactive limb (P < 0.05). CONCLUSIONS Conduit artery responses to exercise were dependent upon the work rate and arm position. The delayed dilation in the heavier exercise, independent of arm position, suggests that stimuli related to the metabolic activity of the distal active skeletal muscle may influence the dimensions of the conduit artery.