Glycerol kinetics were evaluated during high-intensity exercise in five untrained and five endurance-trained subjects. Glycerol rate of appearance (Ra) in plasma was determined by infusing [2H5]glycerol during rest and 60 minutes of cycle ergometer exercise performed at 70% V̇O2 peak. Mean plasma glycerol concentration was greater in trained than untrained subjects throughout exercise (P < .05). The average glycerol Ra during exercise and the integrated lipolytic response to exercise, expressed as total glycerol Ra above baseline, were both greater in trained (7.85 ± 0.72 μmol · kg-1 · min-1 and 289 ± 50 μmol · kg-1 · h-1, respectively) than in untrained (5.68 ± 0.90 μmol · kg-1 · min-1, and 198 ± 31 μmol · kg-1 · h-1, respectively) subjects (P < .05). We conclude that whole- body lipolytic rates are greater in endurance-trained athletes than in sedentary controls during high-intensity exercise performed st the same relative intensity.