1. Urinary excretion of digoxin-like immunoreactive factor and arginine-vasopressin and other parameters related to salt and water metabolism were studied in hyper- and hypo-thyroid rats after different tests. 2. Urinary excretion of arginine-vasopressin was increased in hyperthyroid and reduced in hypothyroid rats with respect to controls, in response to water deprivation or a hypertonic saline load. 3. Control and hypothyroid rats showed the highest urinary excretion of digoxin-like immunoreactive factor after a hypertonic saline load. However, hyperthyroid rats had the highest urinary levels of digoxin-like immunoreactive factor under normal conditions. 4. From these results it is suggested that: (a) hyper- and hypo-thyroid rats exhibit hyper- and hypo-responsiveness of arginine-vasopressin secretion to osmotic stimuli, respectively; (b) an unidentified digoxin-like immunoreactive factor measured in unextracted rat urine may be related to diuresis and natriuresis in control and hypothyroid rats; however, dissociation between this factor and natriuresis is observed in hyperthyroid rats.