The syndrome of inappropriate antidiuretic hormone (SIADH) is characterised by euvolaemic hyponatraemia. Patients with SIADH continue to drink normal amounts of fluid, despite plasma osmolalities well below the physiological osmotic threshold for the onset of thirst. The regulation of thirst has not been previously studied in SIADH. We studied the characteristics of osmotically stimulated thirst and AVP secretion in 8 subjects with SIADH and 8 healthy controls, and the non-osmotic suppression of thirst and AVP during drinking, in the same subjects. Subjects underwent a 2-hour infusion of hypertonic (855 mmol/l) sodium chloride solution, followed by 30 minutes of free access to water. Thirst rose significantly in both SIADH (1.5 ± 0.6 to 8.0 ± 1.2cm, p< 0.0001) and controls (1.8 ± 0.8 to 8.4 ± 1.5cm, p<0.0001), but the osmotic threshold for thirst was lower in SIADH (264 ± 5.5 vs 285.9 ± 2.8mOsm/kg, p<0.0001). SIADH subjects drank similar volumes of water to controls following cessation of the infusion (948.8 ± 207.6 vs 1091 ± 184mls, p=0.23). The act of drinking suppressed thirst in both SIADH and controls, but did not suppress plasma AVP concentrations in SIADH compared to controls (p = 0.007). We conclude that there is downward resetting of the osmotic threshold for thirst in SIADH, but that thirst responds to osmotic stimulation, and is suppressed by drinking, around the lowered set-point. In addition, we have demonstrated that drinking does not completely suppress plasma AVP in SIADH.