This study investigated the individual and combined effects of water and carbohydrate ingestion during prolonged cycling on maximal neuromuscular power (P_max), thermoregulation, cardiovascular function, and metabolism. Eight endurance-trained cyclists exercised for 122 min at 62% maximal oxygen uptake in a 35°C environment (50% relative humidity, 2 m/s fan speed). P_max was measured in triplicate during 6-min periods beginning at 26, 56, 86, and 116 min. On four different occasions, immediately before and during exercise, subjects ingested 1) 3.28 ± 0.21 liters of water with no carbohydrate (W); 2) 3.39 ± 0.23 liters of a solution containing 204 ± 14 g of carbohydrate (W+C); 3) 204 ± 14 g of carbohydrate in only 0.49 ± 0.03 liter of solution (C); and 4) 0.37 ± 0.02 liter of water with no carbohydrate (placebo; Pl). These treatments were randomized, disguised, and presented double blind. At 26 min of exercise, P_max was similar in all trials. From 26 to 116 min, P_max declined 15.2 ± 3.3 and 14.5 ± 2.1% during C and Pl, respectively; 10.4 ± 1.9% during W (W > C, W > Pl; P < 0.05); and 7.4 ± 2.2% during W+C (W+C > W, W+C > C, and W+C > Pl; P < 0.05). As an interesting secondary findings, we also observed that carbohydrate ingestion increased heat production, final core temperature, and whole body sweating rate. We conclude that, during prolonged moderate-intensity exercise in a warm environment, ingestion of W attenuates the decline in P_max. Furthermore, ingestion of W+C attenuates the decline in maximal power more than does W alone, and ingestion of C alone does not attenuate the decline in P_max compared with Pl.