Ten trained male runners performed a treadmill exercise test at 80% VO2max under two experimental conditions, carbohydrate (CHO, 7% carbohydrate) and placebo (P), to determine the effect of carbohydrate ingestion on endurance performance (treadmill run time), blood glucose concentration, respiratory exchange ratio (RER), and subjective ratings of perceived exertion (RPE). Treatment order was randomized and counterbalanced and test solutions were administered double-blind. Ingestion took place 5 min preexercise (250 ml) and at 15-min intervals during exercise (125 ml). Performance was enhanced by 29.4% (p < 0.05) during CHO (115 +/- 25 min) compared to P (92 +/- 27 min). Blood glucose concentration was significantly greater during CHO (5.6 +/- 0.9 mM) relative to P (5.0 +/- 0.7 mM). There was a significant increase in mean RER following CHO ingestion (.94 +/- .01) compared to P (.90 +/- .01). Average RPE was significantly less during CHO (14.5 +/- 2.3) relative to P (15.4 +/- 2.4). These data suggest that time to exhaustion of high-intensity treadmill exercise is delayed as a result of carbohydrate ingestion and that this effect is mediated by favorable alterations in blood glucose concentration and substrate utilization.