Arterial whole blood levels of amino acids (AA) were determined in patients with chronic renal failure (CRF) and in healthy volunteers before and for 75 min after the ingestion of an AA mixture simulating the AA content of an animal-protein meal. In CRF patients, total AA increased more than in control subjects as a consequence of an exaggerated rise in nonessential AA (+86%), mainly glutamine, proline, glutamate, serine, glycine, and alanine. Total essential AA in patients increased as much as in control subjects; however, threonine and phenylalanine showed greater increases while leucine had a smaller increase. As a consequence of the observed alterations, a striking unbalance in the postprandial pattern of arterial AA ensued in CRF patients. The flow of AA to all the organs is altered during the absorptive phase, which is crucial for body nitrogen-pool replenishment.