1. Livers from fed rats were perfused in situ with whole rat blood containing glucose labelled uniformly with (14)C and specifically with (3)H at positions 2, 3 or 6. 2. When ethanol was infused at a concentration of 24mumol/ml of blood the rate of utilization was 2.8mumol/min per g of liver. 3. Ethanol infusion raised perfusate glucose concentrations and caused a 2.5-fold increase in hepatic glucose output. 4. Final blood lactate concentrations were decreased in ethanol-infused livers, but the mean uptake of lactate from erythrocyte glycolysis was unaffected. 5. Production of ketone bodies (3-hydroxybutyrate+3-oxobutyrate) and the ratio [3-hydroxybutyrate]/[3-oxobutyrate] were raised by ethanol. 6. Formation of (3)H(2)O from specifically (3)H-labelled glucoses increased in the order [6-(3)H]<[3-(3)H]<[2-(3)H]. Production of (3)H(2)O from [2-(3)H]glucose was significantly greater than that from [3-(3)H]glucose in both control and ethanol-infused livers. Ethanol significantly decreased (3)H(2)O formation from all [(3)H]glucoses. 7. Liver glycogen content was unaffected by ethanol infusion. 8. Production of very-low-density lipoprotein triacylglycerols was inhibited by ethanol and there was a small increase in liver triacylglycerols. Very-low-density-lipoprotein secretion was negatively correlated with the ratio [3-hydroxybutyrate]/[3-oxobutyrate]. Perfusate fatty acid concentrations and molar composition were unaffected by perfusion with ethanol. 9. Ethanol decreased the incorporation of [U-(14)C]glucose into fatty acids and cholesterol. 10. The concentration of total plasma amino acids was unchanged by ethanol, but the concentrations of alanine and glycine were decreased and ([glutamate]+[glutamine]) was raised. 11. It is proposed that the observed effects of ethanol on carbohydrate metabolism are due to an increased conversion of lactate into glucose, possibly by inhibition of pyruvate dehydrogenase. The increase in gluconeogenesis is accompanied by diminished substrate cycling at glucose-glucose 6-phosphate and at fructose 6-phosphate-fructose 1,6-bisphosphate.