Changes in the neurohypophysial content of arginine vasopressin (AVP) and neurosecretory material in different states of hydration have been reported by many authors (see Jones & Pickering, 1969; Vilhardt, 1970). The present paper reports the effect of hydration on pituitary and plasma levels of AVP and neurophysin in the rat, and on the release of these two peptides in response to haemorrhage. Four groups of male Wistar rats were studied over a period of 1 week. One group was maintained on an unrestricted water intake (control), another on a restricted water intake, the third on 1·8% sodium chloride solution and a fourth group was hydrated (water intake equivalent to 25% body weight/24 h). The rats used weighed 200 g and at least six animals were included in each group. After 1 week the animals were anaesthetized with sodium pentobarbitone (3 mg/100 g) and 0·8 ml blood removed for the determination