The purpose of this study was to determine the effect of carbohydrate supplementation on finishing time of a realistically simulated 80 mile bicycle time trial. Fourteen trained cyclists pedalled at self-selected pace on their own bicycles on windload simulators. Two trials were performed one week apart. Each trial was preceded by two days of prescribed diet, with a final feeding 3-4 h prior to exercise. Following each 10 mile segment, subjects ingested either a non-caloric placebo (PL) or the carbohydrate maltodextrin supplement (MD; 5% maltodextrin +2% fructose) at dosage of 0.25 g.kg-1 body weight (mean intake rate, total: 37 g.h-1, 148 g) in a double blind, counter-balanced design. Mean (+/- SEM) finishing times were faster by 5% with MD (241.0 +/- 2.1 minutes versus 253.2 +/- 2.1 minutes), p < 0.05. With MD, serum glucose levels rose sharply after 40 miles and were significantly higher than PL (p < 0.05). Final glucose values were 6.0 +/- 0.2 mmol.l-1 and 4.1 +/- 0.2 mmol.l-1 for MD and PL, respectively. With MD, free fatty acid levels were significantly lower (p < 0.05), and riders sustained higher average intensity over the entire distance. At the finish, intensities were 64.7 +/- 1.9% VO2max and 55.3 +/- 1.9% VO2max for GP and PL, respectively. Mean carbohydrate oxidation was significantly higher (p < 0.05) for MD, 2.01 +/- 0.2 g.min-1 and 1.64 +/- 0.2 g.min-1 for MD and PL, respectively. These data show that ingestion of MD, during ultraendurance exercise, in comparison to PL, decreases the time to finish a self-paced bicycle time trial.