Oral rehydration during exercise in a hot environment helps athletes to prevent overheating. It is unclear at which exercise intensity the benefits of rehydration are larger. It could be hypothesized that the higher the exercise intensity and thus hyperthermia the larger potential benefit from rehydration. PURPOSE To determine the thermoregulatory and cardiovascular effects of oral rehydration during exercise in the heat in trained and untrained subjects at different exercise intensities. METHODS: Ten endurance-trained (Tr) and ten untrained (Untr) men pedalled in a hot-dry environment (36°C; 23 % rh; 2.5 m·s-1 wind speed) at three intensities (40, 60, and 80% of VO2peak). Subjects performed the same total work (658±53 kJ for Tr and 508±35 kJ for Untr; P<0.05) in all trials by varying exercise time (120, 60 and 45 min, respectively). Trials were performed replacing 80% of fluid losses with a 6% CES (REH) or without fluids (DEH) in a random order. Rectal temperature, whole body sweat rate, forearm skin blood flow (SkBF), and heart rate were monitored. Two-way ANOVA was used to detect differences. RESULTS.TableCONCLUSION: REH (80% of fluid losses) has a cooling effect (lower rectal temperature) during low and moderate exercise intensities (40–60% VO2peak) in both Tr and Untr, but it is not effective at 80% VO2peak in either group despite an increased sweat rate. However, Tr subjects show cardiovascular benefits (lower heart rate) from REH when exercising at high intensity in the heat. Supported by a grant from the Gatorade Sports Science Institute