Since the first studies of the diuresis after relief of urinary-tract obstruction by Wilson, Reisman, and Moyer (1), this phenomenon has received much attention. Earley (2) reported a patient in whom impaired response to anti-diuretic hormone appeared to be the major cause of the diuresis. Bricker and associates (3) studied four patients in whom impaired proximal tubular sodium reabsorption was prominent. Maher, Schreiner, and Waters (4) more recently described a patient in whom they showed that diuresis was due to the osmotic effect of urea. In the present report, observations are presented that indicate that the diuresis that followed the