1. Glycerol and dihydroxyacetone, both antiketogenic and readily metabolized, but differing in their effects on the redox state of the hepatic NAD couples, were given to starved rats and the contents of metabolites were measured in freezeclamped liver and in the blood. The object was to study the effects of changes in the redox state and of the availability of oxidizable substrates on the rate of ketone-body formation. 2. Intramuscular administration of dihydroxyacetone, glycerol or glucose to starved rats decreased the concentrations of acetoacetate and 3-hydroxybutyrate in the blood by 70–80% within 60min., whereas there was no major change in the free fatty acid concentration. 3. Dihydroxyacetone, but not glucose or glycerol, caused an immediate and sustained twofold increase in the blood lactate concentration. 4. Dihydroxyacetone and glycerol caused a rapid fall in the hepatic concentrations of ketone bodies, dihydroxyacetone being more effective. 5. This decrease was not accompanied by significant changes in the concentrations of acetyl-CoA, long-chain acyl-CoA or free CoA. 6. The hepatic glycerophosphate concentration rose about 40-fold on administration of glycerol, whereas with dihydroxyacetone the increase was only about 50%. The large increase in glycerophosphate concentration after administration of glycerol was completely prevented by pretreatment of the rats with tri-iodothyronine. Triiodothyronine-treated rats showed the same decrease in ketone-body concentrations after administration of glycerol as the untreated rats. 7. Glycerol and dihydroxyacetone caused an increase in the hepatic lactate concentration; the pyruvate concentration rose only after injection of dihydroxyacetone. 8. Both compounds increased liver glycogen. 9. Calculation of the [free NAD⁺]/[free NADH] ratios indicated that dihydroxyacetone increased the ratio in cytoplasm and mitochondria, whereas glycerol caused a prompt fall in both compartments, followed at 10min. by a slight rise in the mitochondrial compartment. 10. Dihydroxyacetone did not alter the hepatic content of ATP. 11. The findings suggest that the main reason for the antiketogenic effect of glycerol and dihydroxyacetone was a consequence of their ready metabolism and the provision of an increased supply of C₃ intermediates for conversion into oxaloacetate. Under the test conditions, neither the hepatic content of α-glycerophosphate nor the redox state of the NAD couples appeared to play a major role in the regulation of ketogenesis.