This study investigated the relationship between runners' perceptions of fluid needs and drinking behavior under conditions of compensable heat stress (ambient temperature = 20.5 +/- 0.7 degrees C, 68.9 degrees F; relative humidity = 76.6%). Eighteen experienced runners (15 men, 40.5 +/- 2.5 y, and 3 women, 42 +/- 2.3 y) were given ad libitum access to a sports drink (6% carbohydrate-electrolyte solution) at Miles 2, 4, 6, and 8. After the run (75.5 +/- 8.0 min), subjects completed questionnaires that required them to estimate their individual fluid intake and sweat loss. Dehydration averaged 1.9% +/- 0.8% of initial body weight (a mean sweat loss of 21.6 +/- 5.1 mL.kg-1.h-1). Subjects replaced only 30.5% +/- 18.1% of sweat loss and underestimated their sweat loss by 42.5% +/- 36.6% (P <or= 0.001). Subjects' self-estimations of fluid intake (5.2 +/- 3.2 mL.kg-1.h-1) were not significantly different from actual fluid intake (6.1 +/- 3.4 mL.kg-1.h-1) and were significantly correlated (r = 0.63, P = 0.005). The data indicate that even under favorable conditions, experienced runners voluntarily dehydrate (P <or= 0.001), possibly because they are unable to accurately estimate sweat loss and consequently cannot subjectively judge how much fluid to ingest to prevent dehydration.