When hyperthermia is avoided by fluid ingestion, the addition of carbohydrate to fluid improves performance1. However, rehydration drinks with a level of carbohydrate of 8% could delay gastric emptying during prolonged cycling exercise2 and impair performance. PURPOSE: To test the effects of commonly used rehydration beverages on cycling and muscle performance during prolonged exercise in the heat. METHODS: Seven trained and heat acclimated cyclists (VO2max= 4.3±0.5 L·min−1) pedalled for 120 min at 63% VO2max in a hot-dry environment (36°C; 29% rh; 1.9 m-s−1 wind speed) either without rehydration (CON) or rehydrating 90% of sweat losses with a 0% CES (WATER) 6% CES, 8 % CES or a 8% CES with low electrolyte content (i.e., 8% CESlow). Peak cycling power was measured every 30 min of exercise. Prior and immediately after exercise leg maximal isometric voluntary contraction (MVC) was assessed. During the MVC electrical stimulation was superimposed to calculate central activation ratio (CAR; an index of central nervous system fatigue). Besides performance, cardiovascular and thermoregulatory indexes were measured. RESULTS: Subjects dehydrated by 3.7 ± 0.2% when no drinking (CON) and sweat rate was enhanced when drinking without differences among rehydration trials. Rectal temperature rose to 39 ± 0.1°C after 120 min of CON while remained at 38.3 ± 0.1°C in the rest of the trials. Plasma osmolality and sodium concentration were reduced only during the WATER trial (2.44 L in 90 min). We did not find a difference on cycling peak power among any of the trials at any time. However, knee extensor maximal isometric voluntary contraction declined from its initial values after CON, (7.7 ±1.6 kg), WATER (4.0 ± 1.1 kg) and 8% CESlow (4.1 ± 1.5 kg) while it was maintained with 6% and the 8% CES. Central activation ratio (index of central nervous system fatigue) revealed the same pattern of response. CONCLUSION: Rehydration drinks that vary in a range of 0% to 8% in carbohydrate concentration function similarly to prevent hyperthermia and cardiovascular drift during prolonged exercise in the heat. However, there is a dangerous tendency for WATER to lower plasma sodium concentration when replacing 90% of sweat losses in trained acclimated cyclists. Ingestion of a 6% or 8% CES preserves muscle force after 120 min of continuous exercise in comparison to ingestion of WATER, or a 8% CES with half of the sodium content. Not only carbohydrates ingestion but also electrolytes may play an important role in preserving muscle function after prolonged exercise in the heat that provokes high sodium sweat losses. Supported by GSSI grant.