The camel has been found to be able to regulate the urea excretion on the renal tubular level. The same was demonstrated in sheep. Lactating or growing sheep, when placed on a low protein diet, decreased the fraction of filtered urea appearing in the urine to very low values. The low urea excretion was characterized by a urea U/P ratio not exceeding 5–7 even at low urine flows. The degree to which urea is concentrated in the urine over the plasma at low urine flows was found to be a good indicator of the regulatory state of the kidney. Changes in this regulatory state could be brought about within approximately one hour. A sudden lowering of the maximum urea U/P was found after a deficient protein-free diet was supplemented with grain. Conversely, a urea infusion raised the maximum urea U/P ratio abruptly in a sheep maintained on a low protein intake. The effect of administration of hydrocortisone or ACTH was studied to determine if these hormones could have a direct effect upon the regulatory state of the kidney. No acute effect was observed. Prolonged administration had in some instances a renal effect. The effect, however, was probably secondary to changes in nitrogen metabolism. The main results are that the excretion of urea is regulated by the renal tubule, that changes in the regulation can take place rapidly following a suitable stimulus, and that these changes are independent of plasma urea level.