Water excretion in 16 patients with primary hypothyroidism was lower in amount and more delayed (P less than 0.05) than that in 5 normal controls after acute water ingestion. The mean plasma osmolality of the patients was lower than that of normal controls both before and after water loading. However, the mean urine osmolality was not decreased but rather elevated. The clearance of free water in the patients was lower than that in normal controls before loading, Although the clearance of osmolality in the patients was higher than that in the controls, the difference was insignificant. These two clearance rates were lower than those in the controls after loading. The fractional excretion of sodium (P less than 0.05) and chloride (P less than 0.01) in the patients was significantly higher than that in the controls before loading and both of them remained elevated after loading. Most of the parameters mentioned above improved in 9 patients after treatment with desiccated thyroid.