Depressed food consumption is an early response of experimental, animals to: 1) a dietary deficiency of either protein or an individual indispensable amino acid; 2) a distortion of the dietary pattern of amino acids when protein intake is low; and 3) a substantial elevation in the protein content of the diet. In each of these conditions the change in feeding behaviour is associated with alterations in concentrations of amino acids in blood but in none of them has the biochemical basis for the depressed food intake been established. The depressed food intake of rats consuming a low protein diet in which an imbalance of amino acids has been created by adding quantities of amino acids other than the one most limiting for growth, is associated with elevations in the plasma concentrations of amino acids added to create the imbalance and usually with a depression in the plasma concentration of the growth-limiting amino acid. These changes, in turn, are associated with depression of the concentration of the growth-limiting amino acid in the brain free amino acid pool. Studies in which uptake of amino acids into brain slices has been examined support the conclusion that various distortions of the plasma amino acid pattern, as the result of dietary imbalances of amino acids, can lead to depletion of the brain pool of a specific amino acid through competition between it and other amino acids in surplus in plasma for uptake into brain. The results of studies with rats in vivo of the effects on brain amino acid pools of ingestion of diets containing supplements of amino acids that compete with the growth-limiting amino acid for uptake into brain also support this conclusion. Depletion of the brain pool of the limiting amino acid as the result of feeding a diet with an amino acid imbalance can be related to overall body protein metabolism. In the young growing animal, protein synthesis is stimulated after a meal. Thus, when the diet is limiting in a single amino acid, that amino acid will be depleted from the circulating body pool. At the same time, the activities of amino acid degrading enzymes are low in animals fed a low protein diet; hence, such animals have limited capacity to degrade surpluses of amino acids. These conditions, depletion of the blood pool of the limiting amino acid and slow removal of surpluses of competing amino acids from the blood, will increase the extent of competition between other amino acids and the limiting amino acid for uptake into brain.(ABSTRACT TRUNCATED AT 400 WORDS)