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1. The neurones of the supraoptic nucleus (SON) and paraventricular nucleus (PVN) were stimulated electrically in lactating cats under chloralose anaesthesia. Milk-ejection responses were used to monitor the release of oxytocin and vasopressin and both hormones were assayed in samples of blood collected during stimulation. The position of the tip of the stimulating electrode was confirmed from brain sections stained selectively for cystine-rich neurosecretory material.

2. A previous finding that stimulation of the SON in the cat releases vasopressin without oxytocin was confirmed.

3. Stimulation of the PVN caused both hormones to be released. The ratio of their concentrations in blood was variable; this suggests release from separate neurones.

4. Both hormones were also released on stimulation of the median eminence but not of the zone lying vertically between this structure and the PVN. No neurosecretory material was detected in this zone. These findings argue against the existence of a direct or medial paraventriculo-hypophysial pathway running downwards along the wall of the third ventricle.

5. Study of sections from unstimulated brains confirmed that the tractus paraventricularis cinereus of Greving which runs ventro-laterally from the PVN towards the SON, represents the principal efferent pathway for neurosecretory fibres from the PVN.

6. The results are discussed in relation to the problem of the independent release of oxytocin and vasopressin in response to physiological stimulation of the neurohypophysis.