The neurohypophyseal hormones of three species of Australian marsupials belonging to the family Macropodidae (Macropus rufus, Macropus giganteus, Macropus robustus). have been investigated. In these species three active peptides have been disclosed, one with oxytocic activity and two with pressor activity. The three hormones are present in all the individual glands which have been examined. Purification has been undertaken by ion‐exchange chromatography and paper chromatoelectrophoresis. The more abundant of the pressor hormones (about 80% of the total pressor activity) has been identified as lysine‐vasopressin by its biological properties, its chromatographic behaviour and its amino acid composition. The second pressor principle, more basic, has not yet been chemically characterized but its pharmacological properties and its chromatographic behaviour suggest that it is closely related or identical to arginine‐vasopressin. Dimorphism of vasopressins seems peculiar to Metatheria since it has not been observed in Protheria and very rarely in Eut