T o study the role of circulating natriuretic factors in the postobstructive diuresis that occurs after relief of bilateral, but not unilateral ureteral ligation, cross-circulation was carried out between normal recipient rats and donor rats have either 24-h bilateral (BUL) or unilateral (UUL) ureteral ligation. With BUL donors, there was a rapid marked increase in sodium and water excretion in the recipient rats, sustained for 80-140 min, with a peak approximately 10 times control values. With UUL donors, no significant natriuretic response occurred. Changes in glomerular filtration rate, renal plasma flow, blood pressure, hematocrit, or circulating levels of aldosterone or Pitressin did not explain the diuresis-natriuresis produced by cross-circulation with BUL donors. Differences in the intrinsic renal damage produced by bilateral as compared to unilateral ureteral obstruction did not appear to account for this response, since UUL donors given an acute urea load and urine reinfusion caused a similar diuresis-natriuresis. Moreover, normal donor rats given a urea load also caused a diuresis-natriuresis nearly equal to that produced by BUL rats, and the relationship between increased urea excretion and sodium excretion or urine flow in the recipients was not different in the two groups. Total urine reinfusion for 3 h in donor rats produced a significant, although less marked, diuresis-natriuresis in recipient animals, with only a slight elevation of the blood urea nitrogen level, much less increase in urea excretion rate, and no significant relationship between urea excretion and sodium excretion or urine flow. The results indicate that potent natriuretic factors, which act by decreasing the tubular reabsorption of sodium and water, are present in the blood of rats with bilateral, but not unilateral, ureteral ligation. High blood and urine urea levels appear to be the factors responsible for the marked natriuresis-diuresis occurring in normal rats during cross-circulation with BUL donors, although suggestive evidence of other natriuretic factors in urine reinfused intravenously was also obtained. The data suggest that urea osmotic diuresis is an important mechanism for determining the striking difference between the postobstructive diuresis observed after relief of bilateral as compared to unilateral ureteral ligation.