The [ 14C]2-deoxyglucose (2-DG) technique has been used in conjunction with quantitative autoradiography to determine the metabolic activity of the neurones of the hypothalamo-neurohypophysial system. Water deprivation (4 days) in Wistar rats led to a more than two-fold increase in the metabolic activity of the pars nervosa (PN), and a significant increase in the metabolic activity of paraventricular (PVN) but not supraoptic (SON) neurones. The PN in homozygous Brattleboro rats was significantly more active than that in hydrated Wistar and Piebald Virol Glaxo (PVG) rats; as in Wistar rats, activity of the PN and PVN was significantly increased by 12–15 h water deprivation. The administration of desamino- d-arginine 8 vasopressin (dDAVP) to Brattleboro rats significantly reduced the activity of the PN to that seen in hydrated Wistar and PVG rats. This reduction in activity was accompanied by a significant reduction in plasma osmolality. These results show that (1) in contrast to studies in which the osmotic stimulus was a salt load, water deprivation leads to a significant increase in 2-DG uptake by the perikarya as well as the terminals of neurohypophysial neurones; (2)_increased activity of the neurohypophysial neurones occurs even when there is a genetic deficiency of vasopressin and the activity can be increased further by an osmotic stimulus, and (3) the activity of the neurohypophysial neurones can be significantly reduced by the administration of dDAVP.