Isotope dilution studies of ketone body (KB) turnover have usually been performed using a single 14C tracer and the so called 'combined KB specific activity'. By definition, this approach does not allow to evaluate the individual kinetics of acetoacetate (AcAc) and 3-hydroxybutyrate (R-BHB) which is feasible only using the separate administration of 14C tracer AcAc and R-BHB. In the present study we followed a different approach using the simultaneous administration in vivo of [1,2,13C2] AcAc and m [1,2,3,4(13)C4] R-BHB which allows to evaluate the individual kinetics of the two KB in the some study, thus minimizing the magnitude of blood sampling and the potential changes in the metabolic conditions of each subject. The four isotopic 13C/12C KB ratios of AcAc and R-BHB tracer and tracee blood concentrations along with the fluorimetric measurement of 12C concentrations were determined in each blood sample. Using compartmental analysis following single dose bolus injection the production rate of KB was 206 +/- 57 mumol/min/1.73 m2 (mean +/- SD). The turnover rate of KB using noncompartmental analysis, during continuous infusion in a separate study was 294 +/- 41. The plasma clearance rates of AcAc and R-BHB were 1966 +/- 502 and 1443 +/- ml/min/1.73 m2, respectively. The mean residence time was 17 +/- 3 min and the total distribution volume 20 +/- 9.7 l/m2. We conclude that: (1) stable isotope tracer infusion allows the contemporary in vivo administration of the two KB and the simultaneous assessment of individual AcAc and R-BHB kinetics; (2) the estimated compartmental and noncompartmental parameters of KB turnover were similar to those observed in normal overnight fasting subjects following separate radioactive tracer injections.