1. A method of assaying (14)C in ketone bodies present in blood by using liquid-scintillation counting is described. 2. d(-)-beta-Hydroxy[(14)C]butyrate is converted quantitatively into [(14)C]acetoacetate by means of a coupled oxidoreduction reaction involving NAD(+), d(-)-beta-hydroxybutyrate dehydrogenase and malic dehydrogenase in the presence of a high concentration of oxaloacetate. 3. [(14)C]Acetoacetate is decarboxylated to acetone and carbon dioxide which are trapped separately in a double-well flask and counted subsequently. 4. The method permits the determination of (14)C activity in the individual ketone bodies and allows the activity in the carboxyl carbon atoms of acetoacetate or of d(-)-beta-hydroxybutyrate to be assayed separately from the activity in the remainder of the molecule. 5. Recoveries of (14)C-labelled ketone bodies added to blood approach 100% with good reproducibility in replicate analyses.

1. Rats were starved for 48hr. or fed for 1 week on a high-fat or a high-carbohydrate diet. The effects of these dietary alterations on the rate of production of (14)CO(2) from trace amounts of [U-(14)C]glucose, [1-(14)C]palmitate or [1-(14)C]acetate administered intravenously were studied. 2. The oxidation of [(14)C]glucose was most rapid in the carbohydrate-fed condition and was decreased significantly and to the same extent after starvation and after feeding with fat. 3. Under all dietary regimes studied the maximum rate of elimination of (14)CO(2) from [(14)C]palmitate occurred within a few minutes after injection, but considerably more was oxidized after starvation and feeding with fat than after feeding with carbohydrate. 4. Alterations in diet had no effect on the oxidation and high recovery of administered [(14)C]acetate as (14)CO(2). 5. Graphical analysis showed the presence of several exponential components in the (14)CO(2)-elimination curves. 6. In all studies a marked similarity in oxidative pattern was noted between the starved and the fat-fed rat.

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