The purpose of this study was to determine whether presweetened breakfast cereals with various fiber contents and a moderate glycemic index optimize glucose availability and improve endurance exercise performance. Six recreationally active women ate 75 g of available carbohydrate in the form of breakfast cereals sweetened whole-grain rolled oats (SRO, 7 g of dietary fiber) or sweetened whole-oat flour (SOF, 3 g of dietary fiber) and 300 ml of water or water alone (Con). The meals were provided 45 min before semirecumbent cycle ergometer exercise to exhaustion at 60% of peak O2 consumption. Diet and physical activity were controlled by having the subjects reside in the General Clinical Research Center for 2 days before each trial. Glucose disposal is studied with stable isotope labeled tracers and mass spectrometry. Blood samples were drawn from an antecubital vein for glucose, free fatty acid (FFA), glycerol, insulin, epinephrine, and norepinephrine determinati on. Ther e was no difference in exercise time for the SOF and Con trials. We conclude that eating a meal with a high dietary fiber content and moderate glycemic index 45 min before prolonged moderately intense exercise significantly enhances exercise capacity.