Seven elite marathon runners ran 60 min at 90–96% of their average competition speed, and 4 triathletes and one cyclist bicycled for 2.5 h at 55–65% of their max V02 at 18°C and at 32°C in a climatic room. They drank 400 ml of either redistilled water or a 9% glucose polymer solution before start and every 15 min during exercise. The gastric emptying rate, sweat loss and metabolites in venous blood were measured. The gastric emptying rate limited the replacement of sweat loss. Despite an optimal fluid intake, the resulting dehydration for a whole marathon at 32°C would correspond to about 3% of body weight when drinking water (and 4.5% with glucose polymer) in the elite marathoner. Impaired performance and increased susceptibility to heat illness can be predicted in warm environments.