During dynamic exercise, mean blood velocity (Vmean) in the middle cerebral artery (MCA) demonstrates a graded increase to work rate and reflects regional cerebral blood flow. At a high work rate, however, vasoactive levels of plasma catecholamines could mediate vasoconstriction of the MCA and thereby elevate Vmean at a given volume flow. To evaluate transcranial Doppler-determined Vmean at high plasma catecholamine levels, seven elite cyclists performed a maximal performance test on a bicycle ergometer. Results were compared with those elicited during five incremental exercise bouts and during rhythmic handgrip when plasma catecholamines are low. During rhythmic handgrip the Vmean was elevated by 21±3% (mean±SE), which was not statistically different from that established during moderate cycling. However, at the highest submaximal and maximal work intensities on the bicycle ergometer, Vmean increased by 31±3% and 48±4%, respectively, and this was significantly higher compared to handgrip (P<0.05). During maximal cycling, plasma adrenaline increased from 0.21±0.04 nmol L-1 at rest to 4.18±1.46 nmol L-1, and noradrenaline increased from 0.79±0.08 to 12.70±1.79 nmol L-1. These levels were 12- to 16-fold higher than those during rhythmic handgrip (adrenaline: 0.34±0.03 nmol L-1; noradrenaline: 0.78±0.05 nmol L-1). The increase in Vmean during intense ergometer cycling conforms to some middle cerebral artery constriction elicited by plasma catecholamines. Such an influence is unlikely during rhythmic handgrip compared with low intensity cycling.