The contribution of oxytocin to the maintenance of renal Na+ excretion in the Brattleboro rat has been examined in animals infused with hypotonic saline. Brattleboro rats exhibited hypernatraemia and hyperosmolality associated with greatly increased plasma concentrations of oxytocin by comparison with Long–Evans control rats. Neurohypophysectomy to remove the secretion of the remaining posterior pituitary peptide, oxytocin, led to greatly diminished rates of Na+ excretion in the Brattleboro rat. Oxytocin replacement to achieve plasma levels equivalent to those in intact Brattleboro rats produced a substantial and sustained natriuresis in the neurohypophysectomized animal. Oxytocin secretion evoked in response to saline infusion would thus appear to be effective in promoting renal Na+ excretion in the absence of vasopressin in the Brattleboro rat.