1. Isoprenaline hydrochloride injected subcutaneously or infused intravenously caused drinking and simultaneous antidiuresis in the dog. The minimal effective dose was between 20 and 50 mug per dog.2. Isoprenaline-induced drinking was prevented by the beta-adrenergic antagonist propranolol and enhanced by the alpha-antagonist phentolamine.3. Ganglionic blockade with pentolinium or with hexamethonium did not interfere with the response.4. A delay of 1 hr after injection of 100 mug isoprenaline, before drinking was permitted, did not significantly reduce the amount of water subsequently drunk. A preload of water approximately equal to the volume of water normally drunk caused a greater reduction in water intake in some dogs.5. Isoprenaline caused an increase in heart rate and pulse pressure which lasted for between 1 and 3 hr according to dose. Central venous pressure fell, but mean arterial pressure was little altered. Haemodynamic changes preceded drinking by about 5 min.6. Infusion of isoprenaline caused drinking in the bilaterally nephrectomized dog.7. We conclude that isoprenaline is a potent stimulus to drinking in the dog and that the effect is not exclusively mediated by the renin-angiotensin system.