A new process for preparing the cyclic dodecapeptide triglycyl-lysine vasopressin of the formula ##STR1## which comprises the preparation of a first hexapeptide of formula Boc--Gly--Gly--Gly--Cys(Trt)--Tyr--Phe--NHNH.sub.2 and a second hexapeptide of formula H--Gln(Mbh)--Asn(Mbh)--Cys(Trt)--Pro--Lys(Boc)--Gly--NH.sub.2 by a series of condensations involving the reaction of an appropriately protected peptide unit having an activated carboxyl with an appropriately protected peptide having a free amino group. Subsequently, the first and second hexapeptides are condensed according to the azide coupling method to obtain the linear protected dodecapeptide of formula Boc--Gly--Gly--Gly--Cys(Trt)--Tyr--Phe--Gln(Mbh)--Asn(Mbh)--Cys(Trt)--Pro--Lys(Boc)--Gly--NH.sub.2 ; thereafter the linear protected dodecapeptide is transformed into the desired cyclic dodecapeptide, triglycyllysine vasopressin, by oxidizing and deprotecting processes.