Urine specific gravity is often used to assess hydration status. Athletes who are hypohydrated prior to exercise tend to ingest more fluid during the exercise, possibly to compensate for their pre exercise fluid deficit. The purpose of this study was to evaluate the effect of additional fluid intake on fluid balance and gastrointestinal tract comfort during 1h running in a thermoneutral environment when athletes followed their habitual fluid and dietary regimes. Sixteen men and sixteen women ingested a 6% carbohydrate-electrolyte solution immediately prior to exercise and then every 15 minutes during two runs, with a consumption rate of 2 mL.kg^-1 (LV, lower volume) or 3 mL.kg^-1 (HV, higher volume) body mass. Urine specific gravity and body mass changes were determined before and after the tests to estimate hydration status. During exercise subjects verbally responded to surveys inquiring about gastrointestinal symptoms, sensation of thirst and ratings of perceived exertion. Plasma glucose, heart rate and blood pressure were also evaluated. Men had higher preexercise urine specific gravity than women (1.025 vs. 1.016 g·mL^-1 HV; and 1.024 vs. 1.017 g·mL^-1 LV) and greater sweat loss (1.21 ± 0.27 L vs. 0.83 ± 0.21 L HV; and 1.18 ± 0.23 L vs. 0.77 ± 0.17 LV). Prevalence of gastrointestinal discomfort increased after 45 min. No significant differences on heart rate, rate of perceived exertion, blood pressure or glycemia was observed with the additional fluid intake. From these results it appears that additional fluid intake reduces body mass loss and thirst sensation. When compared to the men, however, preexercise euhydration was more common in women and an increased fluid intake increases the risk of body mass gain and gastrointestinal discomfort.