PURPOSE: To examine the effects of thirst sensation and oropharyngeal reflexes on exercise performance and physiological markers during exercise in the heat. METHODS: Twelve males (mean±SD; age,29±12 years; body mass,74.7±7.9 kg; height,179.4±7.0 cm; maximal oxygen consumption [VO2max],49.8±6.6 ml·kg-1·min-1) performed 90 min of cycling exercise at 55% wVO2max followed by 12 km time trial in the heat (ambient temperature,34.9±0.6 8C; relative humidity,30.3±0.9%; wind speed,3.4 mi·h-1). Two experimental trials were performed in random order; a) participants drank 25 ml of water every 5 min (NT), b) participants were infused with 25 ml of isotonic saline every 5 min via intravenous tube (T). Rectal temperature (Tc) heart rate (HR), and thirst sensation were measured every 5 min throughout the trial. Urine specific gravity (USG), urine osmolality (UOsmo), serum osmolality (SOsmo), and body mass loss (BML) were measured before and after trials to assess hydration status. ? was calculated based on the change from the resting value for each trial. RESULTS: No differences in hydration status between NT and T after trials (USG,NT, 1.012±0.006,T, 1.013±0.007; UOsmo,NT,451±217,T, 467±248 mOsmol; SOsmo,NT,293±4,T 295±4 mOsmol; P>0.05). Also, there were no differences in BML (NT,2.4±0.8%; T,2.3±0.5%; p=0.759). Thirst levels and ? Tc were significantly higher in T than NT after 15 min throughout 90 min of cycling exercise and a 12 km time trial (p=0.05). ? HR was significantly higher in T compared to NT at 55 min to 65 min and before the 12 km time trial started (p<0.05). The time trial was significantly faster in NT (17.7±4.6 min) compared to T (19.6±6.2 min,p=0.048). CONCLUSION: Independent of hydration status, thirst sensation decreases exercise performance and increases Trec and HR. Therefore, frequent fluid intake and drinking before feeling thirsty are as important as the total amount of fluid replacement to optimize exercise performance.