Fat balance is an important determinant of energy balance. Exercise after an overnight fast can significantly increase fat oxidation; however, little information pertaining to the effects of exercise and meal glycemic index on fat oxidation under these conditions is available. The objective of this investigation was to study the acute effects of exercise timing and meal glycemic index (GI) on whole-body fat oxidation. Eight apparently healthy young men completed 4 randomly ordered trials during which measurements were made at rest, during exercise, and for 2 h post-exercise and (or) post-prandial. After an overnight fast, subjects were required to perform 400 kcal (1 kcal = 4.184 kJ) of treadmill exercise (at FATmax) either before consuming a 400 kcal low-GI (Ex-LG) or high-GI (Ex-HG) oatmeal breakfast, or after consuming the low-GI (LG-Ex) or high-GI (HG-Ex) meal. The amount of fat oxidized during exercise was significantly greater during Ex-LG and Ex-HG (17.2 +/- 4.0 and 17.5 +/- 4.7 g, respectively) than during LG-Ex and HG-Ex (10.9 +/- 3.7 and 11.7 +/- 3.5 g, respectively) (p < 0.001), as was the amount of fat oxidized during the entire trial (Ex-LG: 23.4 +/- 4.7 g; Ex-HG: 23.4 +/- 6.5 g; LG-Ex: 18.4 +/- 4.7 g; HG-Ex: 19.6 +/- 4.9 g) (p < 0.05), even though energy expenditure was not different across experimental conditions. No significant effect of meal GI on the amount of fat oxidized was noted. Total fat oxidized during exercise, and for 2 h after exercise, was greatest when morning exercise was performed in the fasted state, independently of meal GI.