PURPOSE: Magnetic resonance imaging (MRI) is becoming established as a modality complementary to computerized tomography and ultrasound for evaluating kidney function. The evaluation of renal function during ureteral obstruction may be improved by contrast enhanced MRI. We evaluated dynamic changes in relative signal intensity in normal and hydronephrotic rat kidneys after intravenous injection of gadolinium (Gd) diethylenetetramine-pentaacetic acid (DTPA). MATERIALS AND METHODS: Using Gd-DTPA enhanced dynamic MRI we evaluated 35, 1-year-old rats, including 10 with partial and complete unilateral ureteral obstruction, respectively, and 15 with sham operated nonobstructed kidneys. Partial obstruction was created in 2-day-old rats by embedding the left ureter into the psoas muscle. Complete obstruction was created 10 days before MRI by placing a ligature around the upper third of the left ureter. MRI was performed before and 0.1 to 60 minutes after and intravenous injection of Gd-DTPA. We calculated relative signal intensity per time interval in the renal cortex, medulla and pelvis. RESULTS: Dynamic relative signal intensity patterns differed significantly among normal, and partially and completely obstructed kidneys. In normal kidneys signal intensity changes were divided into 3 phases after Gd-DTPA administration. During phase 1 rapid parenchymal uptake of Gd-DTPA within the initial minute after injection produced a rapid signal intensity decrease in the cortex, which achieved a minimum of 28% of pre-injection intensity. During phase 2 signal intensity decreased in the medulla and pelvis to a minimum of 18% and 22%, respectively, of pre-injection intensity within 5 minutes. During phase 3 significant relative signal intensity recovery began in the cortex at 7 minutes, and in the medulla and pelvis at 10 to 15 minutes, and lasted 30 to 60 minutes. In partially obstructed kidneys the 3 phases were significantly slower than in controls. In completely obstructed kidneys only phase 1 occurred within 60 minutes. CONCLUSIONS: Gd-DTPA enhanced dynamic MRI provides useful information for distinguishing obstructed from nonobstructed and partially from completely obstructed kidneys.