Six endurance-trained and heat-acclimatized adult males ran for 1 h (or until exhaustion) at room temperature (23.8 degrees C) on three occasions. The work loads approximated 37, 56, and 74% of the subjects' aerobic capacities. Venous blood samples were drawn, and urine was collected before and immediately after each exercise bout. Metabolic cost was partitioned by energy substrate, and metabolic water production was quantified from urinary nitrogen, oxygen, and carbon dioxide production. Total body water loss was recorded as the decrease in body weight during the exercise. All subjects completed 1 h of exercise at the two lower exercise intensities but, due to exhaustion, averaged only 35.5 min at the highest work intensity. There were no significant changes in plasma volume after the exercise bouts. Metabolic water production increased with increasing work intensity as did the fraction of total caloric expenditure derived from carbohydrate metabolism. Plasma protein content significantly increased at all levels of exercise intensity. Metabolic water production alone would be of minimal help in plasma volume maintenance and thermoregulation during endurance exercise.