1. The carotid blood-pressure response to intravenous injection of histamine-free extracts of mammalian posterior pituitary extracts in the platypus (Ornithorhynchus) is described.
2. Pitressin evokes a simple pressor response.
3. Pitocin evokes a simple depressor response.
4. Mixtures of pitressin and pitocin evoke a fall and a subsequent rise.
5. Histamine evokes a depressor response.
6. A preliminary note is made of the simple pressor response to pitressin and depression to pitocin in the deeply-anaesthetized bird and in a lizard.
7. Comparison is made between the responses to posterior pituitary extracts in reptiles, birds, the platypus and eutheria.
8. The possible phylogenetic significance of these observations is discussed.