JH imple and reliable method for the determination of ketone body turnover in vivo using a primed, continuous infusion of [3,4-13C2]acetoacetate is described. Mole percent enrichment of beta-[13C2]hydroxybutyrate and [13C2]acetoacetate is determined by gas chromatography/mass spectrometry using electron-impact ionization and selected ion monitoring. Ketone body flux data are provided from preliminary dog experiments. The method is readily applicable to the study of ketone body metabolism in both laboratory animals and humans.