The aim of this study was to compare the effect of pre-exercise breakfast containing high and low glycemic index (GI) carbohydrate (CHO) (2.5g CHO/kg body mass) on muscle glycogen metabolism. On two occasions, 14 days apart, seven trained men ran at 71%VO₂ max for 30 min on a treadmill. Three hours before exercise, in a randomised order, subjects consumed either isoenergetic high (HGI) or low (LGI) GI CHO breakfasts providing (per 70 kg body mass): 3.43 MJ energy, 175g CHO, 21g protein and 4g fat. The incremental areas under the 3 h plasma glucose and serum insulin response curves after the HGI meal were 3.9- (P < 0.05) and 1.4-fold greater (P < 0.001) respectively, than that after the LGI meal. During the 3 h postprandial period, muscle glycogen concentration increased by 15% (P < 0.05) following the HGI meal but remained unchanged after the LGI meal. Muscle glycogen utilisation during exercise was greater in the HGI [129.1 ± 16.1 mmol/kg dry mass (dm)] compared to the LGI (87.9 ± 15.1 mmol/kg dm, P<0.01) trial. Although the LGI meal contributed less CHO to muscle glycogen synthesis in the 3 h postprandial period compared to the HGI meal, a sparing of muscle glycogen utilisation during subsequent exercise was observed in the LGI trial, most likely as a result of better maintained fat oxidation.