Basal whole body lipid kinetics were evaluated in nine endurance-trained cyclists and 10 untrained healthy controls. The rate of appearance (Ra) of glycerol (an index of whole body lipolysis), the Ra of palmitate (an index of fatty acid release), and the rate of triglyceride-fatty acid cycling (reesterification of fatty acids released during lipolysis) were determined by infusing [2H5]glycerol and [2H2]palmitate in conjunction with indirect calorimetry. All subjects were studied while they were at rest after fasting overnight. Glycerol Ra and free fatty acid Ra in the athletes (7.33 +/- 0.68 and 14.88 +/- 1.35 mumol.kg-1 x min-1, respectively) were two- to threefold higher than the values in untrained control subjects (2.53 +/- 0.15 and 7.64 +/- 0.92 mumol.kg-1 x min-1, respectively; P < 0.02). The total rate of triglyceride-fatty acid cycling was approximately four-fold higher in the athletes (16.86 +/- 2.07 mumol.kg-1 x min-1) than in the control subjects (3.91 +/- 0.36 mumol.kg-1 x min-1). Plasma concentrations of insulin and catecholamines, hormones that regulate whole body lipid kinetics, were the same in both groups. We conclude that resting basal lipid kinetics are markedly increased in athletes involved in strenuous endurance training and that this enhances the potential for increasing fatty acid oxidation rapidly at the onset of exercise.