Objective: The aim of this study was to examine the metabolic responses during 1 h of brisk walking, 3 h after ingesting high glycemic index (HGI) and moderate glycemic index (MGI) breakfasts. Design and subjects: Six females completed three treadmill walking trials (50% O2 max), separated by at least 1 month. Three hours before walking, they ingested either water or a HGI or MGI breakfast. The MGI breakfast consisted of a mixture of an HGI breakfast cereal and low GI carbohydrate (CHO) foods. The GI values of the HGI and MGI meals were 77 and 51, respectively. Setting: The study took place in the School of Sport and Exercise Sciences at Loughborough University, Loughborough, UK. Results: In the HGI and MGI trials, plasma glucose and serum insulin concentrations peaked 15 min into the postprandial period. At the onset of exercise, plasma insulin concentrations were twofold higher in the HGI (31.57.7 l U l-1) than in the MGI trial (15.21.9 l U l-1) (P<0.05). However, there were no differences in substrate utilization between the two CHO trials. Conclusion: These results suggest that although the addition of LGI CHO foods to an HGI breakfast cereal reduces the overall GI of the meal, the metabolic response to exercise is similar to that following a breakfast comprised entirely of HGI foods.