We and others have shown previously the existence of high and low affinity systems for oligopeptide transport in kidney brush border membrane vesicles (BBMV). In the present study we investigated the relationship between the structure of substrates and their affinity for interaction with the high-affinity oligopeptide/H+ transporter in kidney BBMV. Based on competition experiments using [3H]Gly-Gln as a probe we determined the Ki values for more than 60 selected peptides. For a high-affinity interaction with the carrier site the following structural features of substrates are required: (a) both a free amino and carboxyl terminus; (b) the amino group and peptide bond nitrogen located in the alpha-position; (c) a trans peptide bond rather than the cis configuration; (d) L-alpha-amino acid isomers in both COOH and NH2 termini, although D-isomers of hydrophobic amino acids are acceptable in the NH2 terminus; and (e) a backbone of less than 3 amino acid residues. A striking finding of the present study is that, for peptides satisfying these minimal structural requirements, the primary determinant of affinity is hydrophobicity. The fact that there is a highly significant (p less than 0.001) correlation between Ki and hydrophobicity allows the prediction of the affinity for any di- or tripeptide composed of alpha-amino acids in the L-form.