certain octapeptides , which have structures characterized by being a six unit cyclic peptide ring with a dipeptide tail which lacks a glycine unit at position 9 , have potent vasopressin antagonist activity . the compounds here claimed are in general characterized by having an amino acid unit at position 4 which is other than valine . an important species of the group is - 2 -- 4 - α - aminobutyric acid - 8 - arginine - 9 - desglycine ] vasopressin .