To achieve a hyperthermic state 11 subjects exercised at 35 degrees C air temperature in a water-impermeable outfit, until their oesophageal temperature (Tes) exceeded 39 degrees C. Changes of brain stem temperature were assessed by the interspike intervals of auditory evoked potentials, which depend on brain stem temperature. These were recorded at rest before exercise (condition A), after exercise during a period when heat loss from the face was prevented by covering the head with a plastic hood (condition B), and again during face fanning (condition C). An increase in Tes from 37.14 +/- 0.25 degrees C to 39.05 +/- 0.15 degrees C (A to B) produced a significant reduction in interspike intervals, indicating an increase in brain stem temperature. Changing from conditions B to C, Tes and interspike intervals remained constant, indicating no change of brain stem temperature in spite of face fanning. Thus, even in severely heat stressed humans face fanning is not able to lower brain stem temperature significantly below that of the rest of the body core.