Exertional heat illness continues to be a military problem during training and operations. Whereas the hospitalization rate of heat illness is declining, heat stroke has markedly increased.

The changes occurring in the esophageal temperature (Tes) thresholds for initiation of heat loss responses as affected by the circadian period and menstrual cycle were studied. Four women exercised at 60% peak Vo2 in 35 degrees C (ambient water vapor pressure 1.73 kPa) for 30 min at 0400 and 1600 during the follicular (F) and luteal (L) phase. Tes, arm sweating rate (msw), and forearm blood flow (FBF) were measured frequently. At rest, Tes averaged 0.3 degrees C higher during L than F at both 0400 and 1600 and approximately 0.4 degrees C higher at 1600 than at 0400 during both phases. During exercise transients, the slopes of the FBF:Tes and the msw:Tes relationships were not different among treatments. The thresholds for initiation of sweating and cutaneous vasodilation were higher at 1600 than 0400 during both phases. Thresholds during F at 0400 averaged 36.44 degrees C for msw and 36.80 degrees C for vasodilation. The thresholds during L at 1600 averaged 37.46 and 37.53 degrees C for sweating and vasodilation, respectively. Our data indicate that the thermoregulatory effector activity during exercise is a function of numerous inputs, and one of these may be hormonal or hormonal-like in action. Controlling time of day and menstrual cycle phase are as important as controlling for aerobic power, age, and fitness in studying female thermoregulatory responses during exercise.

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The purpose of this study was to test the hypothesis that regulated body temperature is decreased in the preovulatory phase in eumenorrheic women. Six women were studied in both the preovulatory phase (Preov-2; days 9-12), which was 1-2 days before predicted ovulation when 17beta-estradiol (E2) was estimated to peak, and in the follicular phase (F; days 2-6). The subjects walked on a treadmill ( approximately 225 W x m-2) in a warm chamber (ambient temperature = 30 degreesC; dew-point temperature = 11.5 degreesC) while heavily clothed. E2, esophageal temperature (Tes), local skin temperatures, and local sweating rate were measured. The estimate of when the E2 surge would occur was correct for four of six subjects. In these four subjects, E2 increased (P