Five years ago Morinaga reported that the intemal use of the thermal waters of Misasa promoted the action of insuline, inhibited the adrenaline hyperglycaemia and suppressed the alimentary hyperglycaemia. But the highest radon content of the waters used at that time was about 1000×10(-10) Curie units per liter. Recently a more highly radioactive spring was found in Misasa, its radon content being 3000-4000×l0(-10) Curie units per liter, though very feebly mineralized (under 0.5 g per liter) and cold. Therefore the hypoglycaemic effect of this newly discovered water was, investigated in rabbits and compared with the effects of two other thermal waters in Tottori Prefecture and of plain water as a control. Twenty cc. of ten per cent glucofoe solution in thermal waters or in plain water per kilogram of bodyweight was administered to the rabitts by stomach tube and blood sugar level was determined before, 30, 60, 120, and 180 minutes after the administration. All of the three thermal waters decreased the glycaemia significantly as compared the results with plain water control.