Intravenous injection of posterior lobe pituitary extracts into spinal or anæsthetised mammals has several effects. One is to elevate blood pressure. Two animal preparations are commonly used for the biological assay of this activity. Most American workers seem to favour a dog deeply anæsthetised with chloretone or chloralose. 10 per cent, accuracy is claimed. We have no experience of this preparation. There have been very few tracings published and those that we have seen do not inspire confidence. This apart, a method employing dogs is unlikely to find favour in this country because of the difficulty of obtaining, feeding, and housing suitable animals. Most workers in this country use the spinal cat. As originally described by Dale and Laidlaw (1912), the method employing this preparation was insensitive and open to grave errors. Hogben et al. (1924), after intensive study, showed that it could be sensitive, reliable, and susceptible of good discrimination. The Hogben procedure has been generally adopted.