The aim of the present study was to calculate critical closing pressure (CCP) of the cerebral vasculature at rest and during exercise to estimate cerebral vascular tone. Five men and two women were seated upright for 15 min and then performed 15 min of right-legged knee extension exercise at 40, 60 and 75% of their maximal workload (WL(max)). Middle cerebral artery blood velocity (MCA V) and blood pressure in the radial artery were recorded. The CCP was calculated by linear extrapolation from 1 min recordings of pairs of systolic and diastolic pressure and MCA V waveforms from both the right and the left MCA. In both arteries, the CCP increased (right MCA, +6.6 +/- 8.5 mmHg, P = 0.023; left MCA, +7.3 +/- 9.1 mmHg, P = 0.016) during 75% WL(max) without changes in resistance-area product, while femoral vascular resistance of the non-exercising leg decreased (from 0.32 +/- 0.07 to 0.18 +/- 0.05 mmHg min ml(1); P < 0.001). There was no significant difference in CCP between the right and left MCA (P = 0.31). These findings suggest an increase in cerebral vascular tone in both the right and the left MCA from rest to exercise despite a decrease in vascular resistance of the systemic vasculature. In addition, the increases in CCP were related to the increases in plasma noradrenaline concentrations (right, P = 0.001; left, P = 0.025) and decreases in the partial pressure of arterial carbon dioxide (right, P = 0.008; left, P = 0.086), but not to changes in mean arterial pressure (right, P = 0.282; left, P = 0.564) or adrenaline concentrations (right, P = 0.138; left, P = 0.108). We consider that an exercise-induced increase in cerebral vascular tone serves to protect the blood-brain barrier from the exercise-induced hypertension.