PURPOSEThe aim of the present study was to examine the effect of dehydration on exercise performance independently of thirst with subjects blinded of their hydration status. METHODSSeven male cyclists (weight72±9 kg, body fat14±6%, VO2peak59.4±6 mlBULLET OPERATORkg·min) exercised for 2 hours on a cycle ergometer at 55% VO2peak, in a hot-dry environment (35°C, 30% rh), with a nasogastric (NG) tube under euhydrated – non-thirst (EUH-NT) and dehydrated – non-thirst (DEH-NT) conditions. In both trials, thirst was matched by drinking 25 mL of water every 5-min (300 mLBULLET OPERATORh). In the EUH-NT trial sweat losses were fully replaced by water via the NG tube (calculated from the familiarization trial). Following the 2-h of steady state, the subjects completed a 5-km cycling time trial at 4% grade. RESULTSBody mass loss for the EUH-NT and DEH-NT after the 2-h was -0.2±0.6 and -2.2±0.4%, while after the 5-km time trial was -0.7±0.5 and 2.9±0.4%, respectively. Thirst (35±30 vs. 42±31 mm) and stomach fullness (46±21 vs. 35±20 mm) did not differ at the end of the 2-h of steady state between EUH-NT and DEH-NT trials (P>0.05). Subjects cycled faster during the 5-km time trial in the EUH-NT trial compared to the DEH-NT trial (23.2±1.5 vs. 22.3±1.8 km·h, P<0.05), by producing higher power output (295±29 vs. 276±29 W, P<0.05). During the 5-km time trial, core temperature was higher in the DEH-NT trial (39.2±0.7 °C) compared to the EUH-NT trial (38.8±0.2°C; P>0.05). CONCLUSIONThese data indicated that hypohydration decreased cycling performance and impaired thermoregulation independently of thirst, while the subjects were unaware of their hydration status.