Four dodecapeptides of general formula Tyr-Gly-Gly-Phe-Met-X-X-Tyr-Gly-Gly-Phe-Met-NH2 (Enk-X-X-Enk-NH2) possessing X = Arg or Lys have been synthesized and subjected to cleavage by trypsin. The peptide with the sequence containing -Lys-Arg-, depicted as BI-NH2, represents the 100–111 segment of proenkephalin. The time course of the degradation was followed by high performance liquid chromatography. This method allows one to observe the formation of not only the final but also intermediate peptides. Among the peptides studied, the most susceptible to the cleavage was BI-NH2. The primary hydrolysis proceeded rapidly at the arginine residue, followed by slow release of arginine. The other peptides (with -Arg-Arg-, -Lys-Lys- and -Arg-Lys-) were cleaved at both possible positions, but the resulting mixture contained Enk-X as a major product, which was the result of both primary and secondary cleavage.