Chronic partial bladder outlet obstruction was created in mini-pigs by implanting a 6-7 mm ring around the proximal urethra. After a median obstruction period of 63 days the ring was removed, and after a median recovery period of 60 days the animals were sacrificed. At each study occasion the blood flow rate (ml per 100 g per min) was measured by washout of locally injected 133Xe in the bladder wall and the washout curves analyzed by the corrected initial slope method. Stereological estimation of length density and total length of blood vessels in the bladder were performed. The results can be summarized as follows: (1) no differences in the blood flow rate at the time of ring implantation, obstruction, or recovery, (2) no decrease in the blood flow rate in normal bladders after bladder distension, (3) a significant decrease in blood flow rate in obstructed bladders after distension, (4) a 6-fold increase in total bladder blood flow after obstruction, (5) an unchanged blood flow per length of blood vessels per min during the study, (6) no changes in length densities of blood vessels during the study, and (7) a 7-fold increase in total length of blood vessels after obstruction and reversion to control level after recovery. The unchanged blood flow per length of vessels implies that capillary proliferation matched the increased bladder mass during the study.