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 [ 2 H 5 ]glycerol during rest and 60 minutes of cycle ergometer exercise performed at 70% Vo 2 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 at the same relative intensity.