The review of the literature covers the underpinning neurophysiology of temperature regulation and the causes and consequences of fatigue in the heat and the effect of skin cooling (Chapter 1). To date primarily exercise protocols have been used to investigate the effect of high ambient temperatures on fatigue. If the now well established view that the limiting factor during endurance exercise in the heat is a critically high core body temperature, it is possible that this hyperthermia-induced fatigue could be investigated, in part, by passive heating. The purpose of the first experimental chapter of this thesis (Chapter 2) was to determine whether passive heating to increase core body temperature (Te) would have a detrimental effect on subsequent exercise capacity and perceived exertion (RPE) during exercise in the heat and whether head-cooling during passive heating would attenuate the unpleasant sensations of an elevated T, during subsequent exercise in the heat. The study found exercise time following passive heating was reduced and RPE increased, however, RPE was lower following passive heating with head-cooling. Results suggest increased RPE during exercise in the heat is primarily due to the increase in T; Furthermore, head-cooling attenuates the rise in T, and the effect on RPE is proportional to the rise on Te. The results of Chapter 2 are consistent with the large body of research which states endurance exercise in the heat is limited by high core body temperatures. Further research in this area by Nybo and Nielsen (200 1) suggests that alterations in cerebral brain activity may be associated with hyperthermia-induced fatigue during prolonged exercise in the heat. Very few studies have been conducted on non-sedated human subjects to investigate the role of increasing core body temperature per se on cortical electroencephalography (EEG). An elevated core body temperature increases feelings of heat related fatigue and EEG slowing has been associated with a reduced state of arousal or fatigue in sedated hyperthermic humans, furthermore, cooling the skin has been shown to provide relief from feelings of heat related fatigue, Therefore, Chapter 3 investigated whether passive heating to increase in core body temperature by 1°C in conscious human subjects altered cortical EEG and whether face cooling would reverse any changes in the EEG. This study found that increasing core body temperature by passive heating caused a slowing of the EEG. Furthermore the cortical EEG response to face cooling differed depending on the thermal state of the body. There does not appear to be any published literature describing differences in EEG responses to skin cooling depending on the thermal state of the body. Therefore, Chapter 4 developed the learning's from chapter 3 to further investigate the EEG responses to skin cooling at normal as compared to elevated core body temperatures. This study found that non-noxious hand cooling elicits a different cortical response, depending on the thermal state of the body.A limited number of individuals, primarily endurance athletes and military personal are exposed to situations where core temperature is a limiting factor on physical performance. However, a larger number of individuals work at high environmental temperatures that may influence their cognitive or mental performance. Chapter S of this thesis examined the effects of raising skin and core temperature, separately and in combination on the perceptions of heat related fatigue (alertness, contentment, calmness and thermal comfort), cardiovascular function and on objective measures of cognitive performance (reaction time and accuracy). The results of this study suggest that feelings of heat related fatigue and cardiovascular strain can be attributed to a combination of elevated skin and core body temperature, whereas decrements in cognitive performance can be attributed to an elevated core temperature. The final experimental chapter of this thesis (Chapter 6) investigated whether thermal thresholds, that is, the temperature at which we perceive warmth or coolness, are altered by increasing core and skin temperatures and whether the face differs from other body sites in thermal perception sensitivity. Furthermore, menthol has been reported to alter cutaneous thermal sensation, therefore we also investigated whether the topical application of menthol has an effect on cutaneous thermal thresholds at altered skin and core body temperatures or between body sites. Results of Chapter 6 suggest that the perception of warm sensation and cold sensation required a relative increase or decrease in skin temperature of approximately 3-4°C for perception of warmth or coolness, independent of starting cutaneous temperature. The perception of hot pain corresponded to an absolute value of skin temperature in the region of 46°C. The topical application of menthol raised the threshold for the perception of warm sensation and hot pain and lowered the threshold for the perception of cold sensation.