Individuals exposed to extreme heat may experience reduced physiological and cognitive performance, even during very light work. This can have disastrous effects on the operational capability of aircrew, but such impairment could be prevented by auxiliary cooling devices. This hypothesis was tested under very hot-dry conditions, in which eight males performed 2 h of low-intensity exercise ( 30 W) in three trials, whilst wearing biological and chemical protective clothing: temperate (control: 20 degrees C, 30 relative humidity) and two hot-dry trials (48 degrees C, 20 relative humidity), one without (experimental) and one with liquid cooling (water at 15 degrees C). Physiological strain and six cognitive functions were evaluated (MiniCog Rapid Assessment Battery), and participants drank to sustain hydration state. Maximal core temperatures averaged 37.0 degrees C (+/-0.1) in the control trial, and were significantly elevated in the experimental trial (38.9 degrees C +/- 0.3; P <0.05). Similarly, heart rates peaked at 92 beats min(-1) (+/-7) and 133 beats min(-1) (+/-4; P <0.05), respectively. Liquid cooling reduced maximal core temperatures (37.3 degrees C +/- 0.1; P <0.05) and heart rates 87 beats min(-1) (+/-3; P <0.05) in the heat, such that neither now differed significantly from the control trial (P > 0.05). However, despite inducing profound hyperthermia and volitional fatigue, no cognitive degradation was evident in the heat (P > 0.05). Since extensive dehydration was prevented, it appears that thermal strain in the absence of dehydration may have minimal impact upon cognitive function, at least as evaluated within this experiment.