The rate and extent of oxidation of 20 different amino acids has been evaluated as part of a study on partitioning of amino acids in normal lactating cows. Only four amino acids--glutamate, aspartate, alanine and glutamine--were extensively oxidized, and when expressed as a percentage of the injected dose oxidized during 3 h after intravenous injection, the value for these four amino acids was not significantly different from those obtained with the volatile fatty acids. The oxidation of all other amino acids was significantly slower, effectively prolonging their availability for protein synthesis. The pattern of oxidation among the amino acids supports the hypothesis that catabolized protein can provide a protracted source of anaplerotic precursors that augment the metabolic role of the tricarboxylic acid cycle and would have survival advantage in both acute and chronic exigencies. Glutamate and aspartate provide the earliest and largest influx to the tricarboxylic acid cycle, with alanine and, especially, glutamine appearing to serve as reservoirs of amino-nitrogen. Several dispensable amino acids are metabolized as slowly as the indispensable ones, indicating that metabolic conservation is not restricted to the latter group.