Three studies were performed to examine biophysical sources of individual variability in the thermoregulatory responses to exercise, and the influence of skin blood flow on heat loss potential during severe heat stress. Study 1 investigated whether unbiased comparisons of changes in rectal temperature (ΔTre) should be compared at a fixed absolute rate of heat production (Hprod; W) or a fixed Hprod per unit mass (W/kg), and whether local sweat rates (LSR) should be compared at a fixed evaporative requirement for heat balance (Ereq; W) or a fixed Ereq per unit of surface area (W/m2), between independent groups of unequal body mass and body surface area (BSA). Study 2 examined whether individual variation in ΔTre, whole-body sweat loss (WBSL), and steady-state LSR is best explained by biophysical factors related to Hprod, Ereq, and body size, and if factors related to aerobic fitness (VO2max) and body fatness correlate with the residual variance in these responses. Study 3 tested whether alterations in skin blood flow shift the critical vapour pressure (Pcrit) above which core temperature could no longer be regulated in hot/humid conditions, indicating altered heat loss potential from the skin. In study 1, exercise at fixed absolute Hprod and Ereq resulted in greater ΔTre and LSR in smaller individuals (smaller mass and BSA), but exercise at set Hprod in W/kg and Ereq in W/m2 resulted in no differences in ΔTre and LSR, respectively, regardless of body size and %VO2max. In study 2, 50-71% of the individual variation in ΔTre, whole-body sweat loss (WBSL), and steady-state LSR was explained by Hprod (W/kg), absolute Ereq (W) and Ereq (W/m2) respectively, while body fat percentage and %VO2max contributed merely 1-4% to the total variability. In study 3, despite a ~20% lower skin blood flow, Pcrit was unaffected by a large reduction in skin blood flow following iso-smotic dehydration, with no differences in core and skin temperatures and sweating observed. Collectively, these findings suggest that between-group comparisons and modelling of thermoregulatory responses must first consider biophysical factors related to metabolic heat production and body size, rather than factors related to VO2max and body fatness. Furthermore, lower levels of skin blood flow may not impair maximum heat dissipation from the skin to the external environment during severe passive heat stress as previously thought.