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The ability to excrete a water load was studied in Wistar rats and in gerbils (Gerbillus campestris). The rat excreted the entire water load in less than 2 h whereas Gerbillus campestris excreted less than 60% of the water load in 4 h. The gerbils which had received a dose of 15-mu-g/100 g body weight dexamethasone improved their rate of excretion which attained 92 +/- 6% in 2 h 30 min. The andidiuretic hormone (ADH) measured by radioimmunoassay at the time of maximum diuresis was undetectable in rats; in contrast, in gerbils the level of ADH remained relatively high (55.4 +/- 6.7 pg/ml). We conclude that the partial inability of the gerbil’s kidney to excrete a water load is due to a high ADH level and probably to a low concentration of glucocorticoids.