This study evaluated the role of central command in the regulation of common carotid artery blood flow and middle cerebral artery mean flow velocity (V(MCA)) at the onset of arm exercise. Eleven young women performed 2 min voluntary elbow flexion and extension exercise with no load (VOL) that was considered to activate both central command and the muscle mechanoreflex, and 2 min passive elbow flexion and extension exercise (PAS) that was considered to activate only the muscle mechanoreflex. Immediately before the onset of VOL, and V(MCA) began to increase from the baseline and peaked 5 s thereafter (mean +/- s.d.; 20 +/- 5 and 14 +/- 5%, respectively; P < 0.05). Also, VOL increased heart rate (9 +/- 2%; P < 0.05) and cardiac output (16 +/- 3%; P < 0.05). Indexes of the cerebrovascular resistance (MAP/ and MAP/V(MCA)) were reduced at the onset of VOL (13 +/- 4 and 12 +/- 4%, respectively; P < 0.05). However, there were no significant changes in these parameters during PAS. These results suggest that central command plays an important role in the increase of cerebral blood flow at the onset of voluntary exercise.