We examined the relationship between changes in cardiac output (Q̇) and middle cerebral artery mean blood velocity (MCA Vmean) in seven healthy volunteer men at rest and during 50% maximal oxygen uptake steady-state submaximal cycling exercise. Reductions in Q̇were accomplished using lower body negative pressure (LBNP), while increases in Q̇were accomplished using infusions of 25% human serum albumin. Heart rate (HR), arterial blood pressure and MCA Vmean were continuously recorded. At each stage of LBNP and albumin infusion Q̇was measured using an acetylene rebreathing technique. Arterial blood samples were analysed for partial pressure of carbon dioxide tension (Pa,CO2). During exercise HR and Q̇were increased above rest (P < 0.001), while neither MCA Vmean nor Pa,CO2 was altered (P > 0.05). The MCA Vmean and Q̇were linearly related at rest (P < 0.001) and during exercise (P = 0.035). The slope of the regression relationship between MCA Vmean and Q̇at rest was greater (P = 0.035) than during exercise. In addition, the phase and gain between MCA Vmean and mean arterial pressure in the low frequency range were not altered from rest to exercise indicating that the cerebral autoregulation was maintained. These data suggest that the Q̇associated with the changes in central blood volume influence the MCA Vmean at rest and during exercise and its regulation is independent of cerebral autoregulation. It appears that the exercise induced sympathoexcitation and the change in the distribution of Q̇between the cerebral and the systemic circulation modifies the relationship between MCA Vmean and Q̇.