Purpose: The rabbit urinary bladder's early response to partial outlet obstruction includes bladder wall remodeling with marked urothelial and fibroblast hyperplasia (1 day) and smooth muscle hypertrophy (3-5 days) resulting in a 4-5 fold increase in bladder mass within 7 days. In this study, we examined the effect of partial outlet obstruction on bladder blood flow during the initial period of rapid growth (1-7 days).Materials and Methods: Each New Zealand White rabbit was partially obstructed by tying a 2-0 silk ligature loosely around the vesical outlet. After 0 (unoperated), 4 hours, 1, 3, or 7 days of obstruction, 5 rabbits per group were anesthetized and the carotid and femoral arteries cannulated with polyethylene tubing. Additional rabbits receiving sham surgeries were treated like obstructed animals at 4 hours and 1 day post-obstruction (5/group). Using standard methods, fluorescent microspheres were infused through the right carotid artery. Bladder and right kidney were rapidly removed upon completion of sphere infusion; bladder mucosa and muscle were separated. Sphere densities in detrusor, mucosa, and kidney were measured by Interactive Medical Technologies, Ltd. A section of each detrusor tissue was fixed in formalin and immunostained for smooth muscle alpha-actin.Results: Mucosal blood flow (0.20 +/- 0.03 ml./min./gm.) was similar to 4-fold greater than that of detrusor (0.05 +/- 0.01 ml./min./gm.). Sham surgery caused a significant increase in bladder blood flow at 4 hours post-obstruction that returned to control levels by 1 day. Both mucosal and muscle blood flows were slightly higher in rabbit bladders obstructed for 4 hours than in sham-operated rabbits, and substantially greater in those obstructed for 1 day: 0.68 +/- 0.13 ml./min./gm. (mucosa) and 0.26 +/- 0.04 ml./min./gm. (muscle). Blood flows returned to control values by 3 days post-obstruction and remained constant through 7 days. Kidney blood flow was unchanged. Although bladder weight increased 4-fold after 7 days of obstru!