Abstract Concentrations of acetoacetate and β-hydroxybutyrate and their specific radioactivities were shown to be assayable directly in plasma of the rat without the need for prior deproteinization. By utilizing this technique ketone body turnover was estimated in the isotopic steady state and after the single injection of radioactive acetoacetate or β-hydroxybutyrate. While difficulties exist in both types of experiments, the turnover rate of total ketones in the rat was shown to be in the range of 8.1 to 13.2 µmoles per min per 100 g of body weight during starvation ketosis with the true rate probably being closer to the lower figure. The turnover rate in acute diabetic ketosis approximated 15 µmoles per min per 100 g of body weight. The liver was shown to play a central role in interconverting acetoacetate and β-hydroxybutyrate in the intact rat. In functionally hepatectomized animals the capacity to bring radioactive acetoacetate or β-hydroxybutyrate into equilibrium was virtually abolished while in diabetic animals this capability was enhanced.