Whole-body heat loss has been extensively examined during exercise in hot environments. Interestingly, the relationship between predicted heat balance parameters and thermoregulatory strain have been limited to individuals in an euhydrated state. As dehydration has been shown to exacerbate thermal and cardiovascular strain, it is imperative to examine whether dehydration modulates heat exchange during exercise in the heat and its relationship to physiological responses to exercise in the heat. PURPOSE: to examine whether predicted heat balance parameters explain variance in core temperature (Tcore) and heart rate (HR) responses to exercise in the heat following 24 h of fluid restriction. METHODS: 8 participants (4 males and 4 females; age:19±2yrs; VO2max:47.4±6.5ml/kg/min; body mass:61.5±9.4kg; height:166.7±7.5cm) performed a 30-min bout of treadmill running and walking in the heat (32°C, 55% relative humidity) under two conditions: 1) 24 h of fluid restriction (DEH) and 2) euhydration (EUH). Participants exercised at 11W/kg for 15 min followed by 7W/kg for 15 min, which were derived via indirect calorimetry. Tcore was continuously assessed through rectal thermometry and HR was measured at the end of each intensity. Metabolic heat production (M-W), dry heat loss (DHL), and evaporative requirement (Ereq) were calculated for both exercise intensities. Pearson correlation coefficients were utilized to examine the relationship between M-W, DHL, Ereq and Tcore, change in Tcore (Tcore), HR, and change in HR (HR) under DEH and EUH. Linear regressions were performed for all significant correlations. Statistical significance was set at p ≤ 0.05. RESULTS: M-W and DHL were not correlated with Tcore, Tcore, HR, or HR in DEH or EUH at 11W/kg. At 7W/kg under DEH, DHL was negatively correlated with Tcore (R2=0.54, p=0.05) and HR (R2=0.67, p=0.024). All other heat balance parameters were not correlated with physiological responses at 7W/kg in DEH or EUH. CONCLUSION: In this preliminary dataset, Ereq and M-W did not explain variance in physiological responses during DEH. DHL under DEH explained 67% of the variance in post HR and 54% of the variance in Tcore at 7W/kg. Therefore, DHL may contribute to the variation in physiological strain during low intensity exercise in the heat under DEH.