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1. Electrical stimulation of the supraoptico-hypophysial system gave rise to milk ejection from cannulated teats in anaesthetized lactating rabbits. Stimuli applied in the vicinity of the paraventricular nucleus produced signs of sympathetic activity and no milk-ejection responses in the intact animal, but after acute adrenalectomy milk-ejection responses could be elicited from this region.

2. Stimulation of the dorsal, lateral and posterior areas of the hypothalamus, but not of the ventral tuberal regions, caused pupillary dilatation, exophthalmos and hyperpnoea, and inhibited the milk-ejection response to oxytocin injected intravenously 10–15 sec after the end of the stimulus. The inhibitory effect could be simulated by injection of 1–5 µg adrenaline. It was abolished by bilateral adrenalectomy.

3. Inhibition of the milk-ejection response to oxytocin was also produced by stimulation of the splanchnic nerve supply to the adrenal glands, and of the sympathetic nerve supply to the mammary glands. After bilateral adrenalectomy some inhibitory effect on milk-ejection was apparent when oxytocin was injected during prolonged stimulation of the sympathetic centres of the hypothalamus.

4. Adrenaline was 1 to 4 times more active in blocking the milk-ejection response to intravenous oxytocin than noradrenaline.

5. Stimulation of the hypothalamus or mammary sympathetic nerves, and injection of adrenaline or noradrenaline did not inhibit the milk-ejection response to mechanical stimuli applied direct to the mammary glands.

6. The inhibition of the milk-ejection response to oxytocin produced by stimulation of the hypothalamus was associated with inability of the young to remove milk from the mammary glands during suckling.