It has been suggested that a thermal countercurrent exchange may occur in the cerebral vascular bed of humans, thereby creating for the brain a state of relative thermal independence with regard to the rest of the body. However, worrying questions have arisen concerning this suggestion. Experiments were carried out on seven young male volunteers. Hyper- and hypothermic conditions were produced by immersion in water at 38.5 degrees C and 25 degrees C, respectively. During the last few minutes of immersion, the face was cooled or warmed by ventilation with a 200 l.min-1 air flow at 5 degrees C or 40 degrees C, respectively. Internal and peripheral temperatures were recorded. Blood flow in the anastomotic vessels between face and brain was measured by Doppler techniques associated with computerized frequency analysis. The general responses were as classically described, i.e. an increase in peripheral and central temperatures during immersion in the warm bath and a decrease in these variables in the cold bath. The reactions produced by cooling or warming the face were small and easily explained by the direct changes of the heat load they induced. Whatever the thermal conditions, the blood flow in the anastomotic vessels between the vascular bed of the face and that of the brain was never reversed. It was concluded that there was no experimental evidence for an efficient thermal counter-current exchange in the vascular bed of the human head.