1. Rates of appearance and disappearance of total ketone bodies were determined in normal, starved and alloxan-diabetic rats by measuring specific radioactivities and concentrations of blood acetoacetate and 3-hydroxybutyrate at different times after injection of 3-hydroxy[14C]butyrate. 2. The mean rates of appearance were 1·7, 4·2 and 10·9μmoles/min./100g. body wt. respectively for normal, starved and alloxan-diabetic rats. The rates of disappearance were of the same order of magnitude as the rates of appearance. 3. There was a direct correlation between the rates of appearance and disappearance and the blood concentrations of the ketone bodies. 4. The results indicate that in the rat increased ketone-body production is paralleled by increased ketone-body utilization and that the raised ketone-body concentration in the blood in starvation and alloxan-diabetes is due to a slight imbalance between the rates of production and utilization. 5. The findings are discussed in relation to the concept that ketone bodies can serve as fuels of respiration when the supply of carbohydrate is limited.
