The synthesis of a heptadecapeptide corresponding to the amino acid sequence of dynorphin, H-Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg-Pro-Lys-Leu-Lys-Trp-Asp-Asn-Gln-OH, using the 4-methoxy-2, 6-dimethylbenzenesulfonyl (Mds) group for the protection of the guanidino function of arginine, is described. To synthesize dynorphin, three peptide fragments corresponding to residues, 1-3, 4-10, 11-17, were prepared and used as building blocks for the final construction of the full sequence of this opioid peptide. Final deprotection of the fully protected peptide was achieved by treatment with 0.15M methanesulfonic acid in trifluoroacetic acid-thioanisole at room temperature for 1 h and the purification was effected by column chromatography on carboxymethyl (CM)-cellulose and by preparative high performance liquid chromatography (HPLC). The synthetic heptadecapeptide was compared with purified natural dynorphin by means of reverse-phase HPLC and found to be identical with natural dynorphin.