Ca0+ a1Ca z Cs vidual rats: for example, the lowest value recorded during the 48hr. starvation period was a total of 2-13, umoles/g. fresh wt. and the highest value 3 59, umoles/g. fresh wt. The ratio 3-hydroxybutyrate: acetoacetate ranged from 1-7 to 4 9 but was not significantly altered by starvation. However, anoxia led to a large increase in the ratio: for example, when a liver sample was removed 1 min. after killing the animal the 3-hydroxybutyrate: aceto-acetate ratio obtained was 14, whereas a value of 2-8 was found for the same liver by the standard procedure. Similar changes in ratio were observed by Hohorst, Kreutz & Reim (1961) for the cytoplasmic redox couples, and the increase in the ratio 3-hydroxybutyrate: acetoacetate can be interpreted as an increase inthe concentration of mitochondrial NADH due to the anoxia. The total ketone-body concentrations in blood are similar to the values for liver in the 24hr. and 48hr. starved rats, but the ratio 3-hydroxybutyrate: acetoacetate tends always to be lower than the ratio found in liver and this is very marked in the fed animal (Table 1). According to Krebs, Mellanby & Williamson (1962) and Shuster & Doudoroff (1962) the equilibrium constant, K, of the 3-hydroxybutyratedehydrogenase system is 1F42 x 10-9. Substituting a value of 3 0 for the ratio3-hydroxybutyrate: acetoacetate in the equation