1. Intravenous (10 micrograms/kg) or intracisternal (1 microgram/kg) clonidine inhibited the diuretic response elicited by negative pressure breathing (NPB) or left atrial distension (LAD) in chloralose anaesthetized dogs. The drug reduced the induced tachycardia, but not the increase in respiratory rate elicited by NPB. Blood pressure was unchanged. 2. Propranolol (1 mg/kg IV) did not change NPB-induced diuresis. 3. Intravenous yohimbine (1 mg/kg IV) restored these two diuresis inhibited by intravenous or intracisternal clonidine, whereas prazosin (0.05 mg/kg IV) was without effect. 4. These results show that the adrenergic receptor implicated in the volumetric control of vasopressin secretion could be of the alpha 2 subtype. This alpha 2 adrenoceptor could be localized at a central level. 5. The possible clinical consequences (hydrosaline retention and antihypertensive drugs; use of clonidine in aerospace medicine) of these data are discussed.