The determination of the degree and ability of recovery of obstructive hydronephrosis is important in treatment and prediction of prognosis. Among the various methods in determination of the status of kidney, intravenous pyelography is the most inportant tool to evaluate the morphological and functional changes of kidney in vivo. The purpose of this experiemnt was the determination of process and recoverability of hydronephorsis depend on duration of unilateral uretreal obstruction. For the experiment, 32 rats were divided into 2 group: the first group of 12 rats were used to determine the process of hydronephrosis during 1 to 4 weeks of unilateral ureteral ligation and the second group of 20 rats were used to determine the recoverability of hydronephrotic kidney depend on same duration of ureteral ligation following relief of ligation. Intravenous pyelography was undertaken and renal angiography, gross and microscopic examination were added. the results obtained are summarized as follows: 1. Kidney enlargement and pelvic dilatation were progressively increased with a relaion to duration of ureteral obstruction until 4 weeks. 2. Renal excretory function is not impaired until 3 weeks obstruction, but rapidly impaired to nonfunctioning in 4 weeks obstruction. 3. Renal recovery was possible following relief of ureteral ligation within 3 weeks of ureteral obstruction, but histopathological and functional recovery were impossible in 4 weeks of ureteral ligation. 4. There is no direct correlation between duration of obstruction and recoveraility following relief within 3 weeks of uretral obstruction. 5. Intravenous pyelography is a simple and accurate method to determine the degree of obstructive hydronephrosis and to evaluate morphological and functional recovery of kidney following relief of obstruction.