To study the biology of bladder mucosal grafts we developed an animal model using New Zealand white male rabbits. A 25 x 9 mm. segment of bladder mucosa was harvested and tubularized over an 8F catheter using 7-zero polyglactin sutures. An equivalent portion of rabbit urethra was then excised and the graft was anastomosed to this defect in an end-to-end fashion. A urethral catheter was left in place to provide bladder drainage and to stent the anastomosis. Animals were sacrificed on postoperative days 1 to 90. India ink was injected into the aorta at sacrifice to visualize the microvasculature. All 59 specimens were stained with hematoxylin and eosin, and studied using light microscopy. Our results demonstrated vascular ingrowth at 72 hours. Between postoperative days 8 and 10 healthy viable epithelium first bridged the entire urethral defect. By postoperative day 12 the epithelial lining was complete. A poor outcome was observed in all animals whose stents were removed early. We conclude that the biology of bladder mucosal grafts is unique in that the graft initially undergoes partial degeneration followed by regeneration. Of concern are the results of those animals whose stents were removed early. In all such cases a poor outcome was observed.