Intestinal permeabilities of five dipeptidyl derivatives of L-α-methyldopa ( I ) were studied by an in situ intestinal perfusion method. The dipeptides displayed a significant increase in their permeabilities compared to L-α-methyldopa. The increases ranged from 4 to 20 times. These results suggest that the peptide transport system is less structurally specific than the amino acid transport systems and can be used as an absorption pathway for peptide analogues. The kinetic advantage demonstrated by the dipeptide, L-α-methyldopa-L-phenylalanine, over the amino acid analogue, L-α-methyldopa, suggests that the peptide carrier would be a possible route for improving the intestinal absorption of pharmacologically active amino acid analogues. Furthermore, the preliminary results of in vitro hydrolysis studies of selected dipeptidyl derivatives indicate that the peptide carrier system could be used as a base for a prodrug strategy.