The kinetics of the influx of tryptophan and phenylalanine into guinea-pig intestinal rings have been examined. The transfer of these two amino acids can be described by a single transport system, each amino acid having an affinity constant, Kt, of about 4 mM for the influx mechanism. Mutual inhibition studies have shown that the inhibitory constant of each of the amino acids is also 4 mM. Although fully competitive inhibition between the two amino acids occurs, the inhibition of the influx of the amino acids by sugars exhibits kinetics of the "pseudo-competitive" type. Such behaviour is compatible with an allosteric interaction between two different binding sites, one for each class of compounds. The lack of correlation between the inhibitory potency of a given sugar and its rate of transfer, as testified by a comparison of the effects of galactose and beta-methyl-glucoside on phenylalanine influx, can be reconciled with the "allosteric-interaction hypothesis", but specifically repudiates any theory that attempts to explain such interactions in a way that requires such a correlation. The fact that allosteric interactions are retained in cells preloaded with sodium also precludes a primary role for sodium in the mechanism of such interactions.