On two separate occasions six trained subjects (peak oxygen consumption [VO2peak] 4.41/min) rode for 60 min at 70% of VO2peak and then to exhaustion at 90% of VO2peak to determine the effects of mild dehydration on high-intensity cycling performance time in the heat (32 degrees C, 60% relative humidity, wind speed 3 km/h). In one trial (F) subjects ingested a 400 ml bolus of 20 mmol/l NaCl immediately before, and then as repetitive 120 ml feedings every 10 min during the first 50 min of exercise. In the other trial they did not ingest fluid (NF) either before or during exercise. The order of testing was in a counter-balanced random sequence. For the first 60 min of exercise mean (+/- SD) VO2 (2.90 +/- 0.39 vs 2.93 +/- 0.38 l/min) and respiratory exchange ratio (RER; 0.95 +/- 0.03 vs 0.94 +/- 0.04) values were similar between F and NF trials. However, weight loss was significantly reduced during F compared to NF (0.16 +/- 0.39 vs 1.30 +/- 0.22 kg; p < 0.005) and high-intensity cycling time to exhaustion was significantly increased (9.8 +/- 3.9 vs 6.8 +/- 3.0 min; p < 0.005). Increased cycling times to exhaustion in the F trial were not associated with any measurable differences in heart rate (HR), body temperature, respiratory gas exchange, leg muscle power over 5 sec, or the degree to which fluid ingestion reduced the level of dehydration within the group. Only the ratings of perceived exertion (RPE) and plasma anti diuretic hormone (ADH) concentrations were significantly increased in the NF trial compared to the F trial.(ABSTRACT TRUNCATED AT 250 WORDS)