Abstract: Dear Editor-in-Chief: Our study described the continuous intestinal (core) temperature (Tc) and fluid balance responses of 18 runners undertaking a 21-km mass-participation road race in heat (27.2 ± 1.0°C) and humidity (87 ± 5%) (1). We can confirm that Noakes presents an accurate illustration of our data, which revealed nonsignificant relationships between relative body mass loss (i.e., % dehydration) and final Tc, total fluid intake and final Tc, and total fluid intake and average running speed (Figs. 1a-c, respectively). Furthermore, no significant relationship was observed between any fluid-balance variable (e.g., dehydration, total fluid intake) and any Tc variable (e.g., peak Tc, ΔTc, Tc integral). Noakes highlights that our data agree with numerous earlier field-based studies, and our interpretation concurs with his comments: dehydration was not a major determinant of Tc during approximately 2 h of self-paced outdoor running in heat. The suggestion that a dehydration threshold be exceeded before a significant relationship with Tc is observed (2) was not supported by our findings. Although the magnitude of dehydration observed (i.e., 2.8 ± 1.0%; 0.9-3.9%) was less than the average of 4% observed for warm-weather marathon races (2), no significant relationships were observed between final Tc and dehydration > 1% (r2 = 0.20, P = 0.074, N = 17), > 2% (r2 = 0.07, P = 0.42, N = 12), or > 3% (r2 = 0.03, P = 0.67, N = 9). We are cognizant that our findings represent a paradox to evidence from numerous well-controlled laboratory studies establishing that beyond 1 h of fixed-intensity exercise, dehydration elevates Tc and fluid replacement attenuates Tc elevation (6). Noakes highlights several limitations in the experimental design of these studies (e.g., fixed exercise intensities, lack of wind velocity) that future laboratory-based research should address to enhance the ecological validity to outdoor situations. It seems timely to reaffirm that the major determinant of Tc elevation during running is relative exercise intensity (% V˙O2max) (4). The higher the intensity, the lower the critical ambient temperature and water vapor pressure above which Tc does not attain steady state but increases as a linear function of time (3). Prolonged running reduces running economy (8), and hyperthermia reduces V˙O2max (7), independently of fluid balance (7,8). In isolation or in combination, these factors will increase the % V˙O2max for a given running speed and will therefore impact Tc (5). Using the example of a runner with characteristics representative of our sample (V˙O2max = 55 mL·kg−1·min−1, speed = 10.8 km·h−1, intensity = 65% V˙O2max, V˙O2 = 35.8 mL·kg−1·min−1) (8), the combined effects of empirically observed deteriorations in running economy (e.g., 4.2 mL·kg−1·min−1) (8) and V˙O2max (e.g., 16%) (7) would elicit an increase in % V˙O2max at a constant 10.8 km·h−1 from 65 to 87% V˙O2max. The combination of this intensity and the environmental conditions of our study would increase Tc as a linear function of time (3). Such a scenario fittingly describes several of the Tc responses observed in our study. The magnitude of Tc responses we observed are a testament to the thermoregulatory challenge imposed by prolonged exercise in a hot-humid environment, and their diversity provides insight into the complexity of thermoregulation during exercise. Chris Byrne, PhD School of Sport and Health Sciences University of Exeter Exeter, United Kingdom DOI: 10.1249/mss.0b013e31804a805a