Amino acid concentrations were measured in plasma, brain, muscle and liver from rats fed leucine-limiting diets containing varying proportions of other indispensable amino acids (IAA), the branched-chain amino acids (BCAA) and norleucine, a BCAA analog known to compete with large neutral amino acids (LNAA) for transport into tissues. Leucine was low and other IAA were high when dietary IAA were 125% and leucine was 65% of requirements; higher leucine and lower IAA concentrations occurred when dietary IAA were 75% of requirements. Tissue leucine was high and isoleucine and valine were low in rats fed excess leucine. Norleucine induced dose-dependent reductions in BCAA, especially in brain and muscle in which isoleucine or valine were sometimes undetectable. Leucine was not depressed further when control values were low as in the rats fed 125% IAA. Norleucine frequently prevented the high BCAA found after feeding additional BCAA. Other LNAA tended to be low in the brain and muscle of rats fed norleucine. Lysine was high only in the tissues of rats fed 75% IAA and norleucine; this effect was prevented when added leucine was given. Brain tryptophan, but not always serotonin, was low in rats fed norleucine. The results show transport-related, selective and usually marked depletions of tissue BCAA in rats fed norleucine; this suggests norleucine may be an aid in the treatment of clinical conditions involving excesses of BCAA.