Women may be challenged to maintain thermoregulation due to hormonal changes associated with the menstrual cycle. The purpose of this study was to assess the effect of the menstrual cycle phase on core temperature, hydration status, and perceived exertion while exercising under uncompensable heat gain. Eleven eumenorrheic women (24.4 ± 1.1 yrs, 65.7 ± 2.4 kg, 22.7 ± 1.5% body fat) walked for two 180-min trials in a heat chamber (35 °C and 30% relative humidity) during early-follicular (EF) and mid-luteal (ML) phases. Subjects completed three intervals of 50 min of exercise at 50% VO