M edullary collecting duct function was studied by direct microcatheterization techniques in rats undergoing postobstructive diuresis. Significant net addition of water and sodium to the duct was demonstrated during postobstructive diuresis after relief of 24-h bilateral ureteral ligation. This striking abnormality in function was associated with reduced delivery of sodium and water to the collecting duct compared to sham-operated controls. To examine the role of circulating factors in this phenomenon, another group of rats was studied that underwent 24 h of total urine reinfusion into the femoral vein. Natriuresis and diuresis were similar to the postobstructive group, but absolute collecting duct reabsorption of sodium and water was normal. The natriuresis and diuresis in rats with urine reinfusion resulted from increased delivery of fluid and sodium to the medullary collecting duct. A third group of rats was studied with 24-h unilateral ureteral ligation as well as urine reinfusion from the contralateral normal kidney. Without urine reinfusion there was no diuresis-natriuresis but with urine reinfusion the diuresis and natriuresis after relief of unilateral obstruction was similar to that after relief of bilateral obstruction. Moreover, net addition of sodium and no significant water reabsorption were demonstrated in the medullary collecting duct of such animals. The results indicate that (a) the medullary collecting duct is the critical nephron segment affected by ureteral obstruction, since postobstructive diuresis occurred despite reduced delivery of fluid from the more proximal nephron; (b) the net addition of sodium to the medullary collecting duct observed during postobstructive diuresis is probably a direct effect of obstruction, since it was found during postobstructive diuresis after relief of bilateral or unilateral ureteral ligation, but not with urine reinfusion alone; and (c) blood-borne factors are important in the development of postobstructive natriuresis and diuresis, and probably act by increasing the fraction of filtered sodium and water delivered from the proximal and distal tubule to the collecting duct.