Time courses of 13C labels from alanine or lactate and ethanol in perfused mouse livers have been followed by NMR. The enrichment at specific carbons of glucose, glutamate, glutamine, aspartate, acetate, acetoacetate, beta-hydroxybutyrate and lactate has been measured. The specific labeling of glutamate in the presence of labelled alanine and labeled or unlabeled ethanol shows that under these conditions alanine enters the tricarboxylic cycle almost exclusively through pyruvate carboxylation, whereas ethanol is the exclusive source of acetyl-CoA. By comparing the randomization of 13C between C3 and C2 of glutamate it is possible to estimate the mitochondrial fumarase activity; the C6-to-C5 ratios in glucose give the additional scrambling by cytosolic fumarase exchange. These two ratios provide the basis for a continuous study of the activity of the malate-aspartate cycle during the metabolism of ethanol.