A tridecapeptide corresponding to the entire amino acid sequence of chicken neurotensin was synthesized by applying a new deprotecting procedure with trifluoromethanesulfonic acid-thioanisole in combination with a new arginine derivative, Arg (mesitylene-2-sulfonyl). The synthetic avian neurotensin was found to be as active as synthetic bovine neurotensin in terms of contractile activity in isolated guinea-pig ileum and hypotensive effects in rats.