1. In the present study we examined the effect of hyperthermia on the middle cerebral artery mean blood velocity (MCA V(mean)) during prolonged exercise. We predicted that the cerebral circulation would be impaired when hyperthermia is present during exercise and assumed that this could be observed as a reduced MCA V(mean). 2. Eight endurance trained men (maximum oxygen uptake (V(O2,max)) 70 +/- 1 ml min(-1) kg(-1) (mean +/- S.E.M.)) performed two exercise trials at 57 % of V(O2,max) on a cycle ergometer in a hot (40 degrees C; hyperthermic trial) and in a thermoneutral environment (18 degrees C; control trial). In the hyperthermic trial, the oesophageal temperature increased throughout the exercise period reaching a peak value of 40.0 +/- 0.1 degrees C at exhaustion after 53 +/- 4 min of exercise. In the control trial, exercise was maintained for 1 h without any signs of fatigue and with core temperature stabilised at 37.8 +/- 0.1 degrees C after approximately 15 min of exercise. 3. Concomitant with the development of hyperthermia, MCA V(mean) declined by 26 +/- 3 % from 73 +/- 4 cm s(-1) at the beginning of exercise to 54 +/- 4 cm s(-1) at exhaustion (P < 0.001). In contrast, MCA V(mean) remained unchanged at 70-72 cm s(-1) throughout the 1 h control trial. 4. When individually determined regression lines for MCA V(mean) and arterial carbon dioxide pressure (P(a,CO2)) obtained during preliminary exercise tests were used to account for the differences in P(a,CO2) between the hyperthermic and control trial, it appeared that more than half of the reduction in MCA V(mean) (56 +/- 8 %) was related to a hyperventilation-induced drop in P(a,CO2). Declining cardiac output and arterial blood pressure accounted for the remaining part of the hyperthermia-induced reduction in MCA V(mean). 5. The present results demonstrate that the development of hyperthermia during prolonged exercise is associated with a marked reduction in MCA V(mean).