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1. To find how urea contributes to the water-conserving ability of a herbivore's kidney, groups of ten young rabbits on a low-protein diet and at three different levels of dietary electrolyte were given 1·8 g urea by mouth daily for 3 days. Vasopressin was administered daily to half the animals in each group.

2. The urinary osmolarity and urea output of each animal was recorded daily during the urea loading and for a 3-day control period before and after loading. The renal water requirement for non-urea solute output (defined as daily volume/daily non-urea solute output) was calculated. The sodium content of renal cortex and medulla was measured in some animals from each group.

3. Urea caused additional water excretion only in those rabbits which were receiving the low-salt diet. There was invariably increased water excretion when the ratio of urea to non-urea solute output exceeded 2.

4. In most of the rabbits on normal-salt and high-salt intake, urea produced little change in the volume in which non-urea solute was excreted. Three out of the ten high-salt animals showed significant reduction of this volume during urea-loading.

5. Vasopressin significantly reduced the volume required for non-urea solute output, but the effect of vasopressin was independent of urea-loading and of dietary electrolyte level.

6. The low-electrolyte diet significantly reduced the sodium concentration in the rabbits' renal medullary tissue.

7. It is concluded that in rabbits urea contributes to water retention mainly by its high permeability, enhanced by vasopressin, which permits maximal water reabsorption in the renal medulla. Water retention by means of uphill transport of urea, if it occurs at all, is slight.