Abstract: A method for preparing a peptide derivative of formula (I) or a salt thereof: which method comprises the steps of removing two MBzl groups from a compound of formula (II) or a salt thereof: wherein MBzl represents a 4-methoxybenzyl group which serves as a protective group for a thiol group, and R1 and R2 represent hydrogen or a protective group for Trp or Arg respectively; and subsequently oxidizing in an aqueous medium having a pH from 4 to 6 to form an intramolecular disulphide bond; intermediates useful for preparing the compounds of formula (I) and preparation of the intermediates.