1. Nursing and suckling behaviour of rabbits is described, and evidence given that an active process of milk ejection ('let-down') occurs in this as in other species.
2. Intravenous injection of posterior pituitary extracts in anaesthetized rabbits resulted in ejection of milk from a cannulated teat duct. The threshold dose was about 5 mU. and maximal responses were produced by 200 mU. of extract. Whole posterior pituitary extract was more effective than the oxytocic fraction, which was in turn more effective than the vasopressor fraction.
3. Stimulation of the supraopticohypophysial (s.o.h.) tract in anaesthetized rabbits also resulted in ejection of milk from a cannulated duct. Kymographic records of this response were similar to those obtained after injection of appropriate doses of posterior pituitary extract.
4. Lesions in the s.o.h. tract in lactating rabbits caused a marked diminution in the quantity of milk obtained by their litters in standard suckling tests, and incomplete evacuation of the mammary glands. Intravenous injection of posterior pituitary extract (30–200 mU.) into the does immediately before nursing gave a marked increase in the amount of milk obtained by the young and complete evacuation of the mammary glands. Stimulation of the region of the s.o.h. tract in these animals failed to elicit milk ejection from cannulated teat ducts.
5. Rabbits with hypothalamic lesions that did not involve the s.o.h. tract showed a normal milk-ejection reflex when suckled by their young, and a milk-ejection response after electrical stimulation of the s.o.h. tract.