Ketone body kinetics were quantified in six children (3-5 yr old), who were fasted for 13-22 h, by a combined perfusion of [3-13C]acetoacetate ([13C]AcAc) and D-(-)-beta-[4,4,4-2H3]hydroxybutyrate (beta-[2H3]OHB) and gas chromatography-mass spectrometry analysis. Results were analyzed according to the "single-pool" (combined enrichments) or the "two-accessible pools" models. After 20-22 h of fasting, ketone body turnover rate was 30-50 mumol.kg-1.min-1, a rate achieved after several days of fasting in adults. At low ketosis, acetoacetate was the ketone body preferentially synthesized de novo and utilized irreversibly. When ketosis increased, acetoacetate irreversible disposal was not enhanced, since it was largely converted into beta-OHB, whereas beta-OHB irreversible disposal was very much increased. The single-pool and two-pool models gave similar ketone body turnover rates when [13C]AcAc was the tracer, whereas the use of beta-[2H3]OHB gave some more divergent results, especially at low ketosis. These studies demonstrate that ketogenesis is very active in short-term fasted children and that the use of a combined infusion of [13C]AcAc and beta-[2H3]OHB is a convenient way to give insight into individual ketone body kinetics.