Concentrations of several large neutral amino acids (LNAA) were earlier shown to be low, especially in brain, in rats fed a low protein diet containing a mixture of LNAA analogues. The purpose of this study was to learn if individual analogues would induce similar effects. Four hours after first feeding one meal containing norleucine, norvaline, alpha-aminophenylacetic acid, or alpha-aminooctanoic acid, concentrations of branched-chain amino acids were low in plasma, brain, liver and muscle; tyrosine and phenylalanine were more effectively reduced in brain than in other tissues. Lysine and arginine concentrations were low in brains of rats fed the basic amino acid analogue, homoarginine; concentrations of large and small neutral amino acids were unchanged. Dopamine was not low in brains having low tyrosine levels; serotonin was low in rats receiving alpha-aminooctanoate, the only analogue associated with a significant depression in brain tryptophan. The results suggest that the analogues have differing abilities to alter concentrations of tissue components. Decreases, especially in brain amino acid concentrations, may result from selective competition by analogues of a given transport class with natural amino acids transported from blood into brain by the same system.