Transport of [D-Ala2, D-Leu5]enkephalin (DADLE(], Tyr-D-Ala-Gly-Phe (TDAGP) and tyrosine into rat jejunum mucosal cells was investigated in vitro. Active transport of either the pentapeptide (DADLE) or the tetrapeptide (TDAGP) into jejunal villi was not detected. Because substantial degradation of these peptides occurred during incubation, the proteolytic enzyme inhibitors, bestatin (30 microM) or D-phenylalanine (20 mM), were added during uptake studies of DADLE or TDAGP, respectively. The presence of these inhibitors significantly reduced degradation of the oligopeptides; however, no accumulation of peptides occurred in the mucosal tissue. Tyrosine was actively transported by the jejunum mucosal cells demonstrating the viability of the transport mechanisms of this in vitro preparation. The results suggest that there are no active transport systems for enkephalin-like oligopeptides.