In an attempt to establish an in vitro/in situ correlation of intestinal permeability data, the permeability coefficients (Papp) for a series of model peptides, which were determined using an in situ perfused rat ileum model, were compared to the permeability coefficients (Pmono) determined using an in vitro cell culture model (Caco-2). The model peptides, which were all blocked on the N-terminal (acetyl, Ac) and the C-terminal (amide, NH2) ends, consisted of D-phenylalanine (F) residues (e.g., AcFNH2, AcFFNH2, AcFFFNH2). To alter the degree of hydrogen bonding potential, the nitrogens of the amide bonds were sequentially methylated [e.g., AcFF(Me)FNH2, AcF(Me)F(Me)FNH2, Ac(Me)F(Me)FNH2, Ac(Me)F(Me)F(Me)]. These peptides were shown not to be metabolized in the in situ perfused rat ileum system. The results of the transport experiments showed that there were poor correlations between the apparent permeability coefficients (Papp) determined in an in situ perfused rat ileum model and the octanol-water partition coefficients (r = 0.60) or the hydrogen bonding numbers (r = 0.63) of these peptides. However, good correlations were observed between the in situ Papp values for these peptides and their partition coefficients in heptane-ethylene glycol (r = 0.96) and the differences in their partition coefficients between octanol-water and isooctane-water (r = 0.86). These results suggest that lipophilicity may not be the major factor in determining the intestinal permeability of these peptides and that hydrogen bonding potential may be a major contributing factor. These results suggest that lipophilicity may not be the major factor in determining the intestinal permeability of these peptides and that hydrogen bonding potential may be a major contributing factor.(ABSTRACT TRUNCATED AT 250 WORDS)