The effects of urinary-tract obstruction on renal function have been clarified using modern physiologic techniques in animal models. These effects are obviously dependent on the severity and duration of obstruction, together with the presence or absence of extrarenal factors, particularly uremia, as exemplified by the differences in renal function between bilateral and unilateral obstruction. The beneficial effects on the unilateral postobstructive kidney of volume expansion (5) and the importance of azotemia in determining POD (20, 21) have possible clinical applications. Further experimental work is required to understand the mechanism responsible for the altered renal hemodynamics and decreased GFR, as well as the abnormalities in medullary function resulting from obstruction. Improvement in the management of this common clinical problem may then be forthcoming.