Bladder overdistension has been found to cause transitional morphological changes in innervation which correlate with changes in micturition and bladder contractility. We investigated the influence of overdistension on bladder wall morphology using histological and enzyme histochemical methods. Overdistension was induced in female rats for 3 h by forced diuresis and balloon obstruction of the bladder neck. Oedema was seen beneath the mucosa at 12 h, with hyperemia and haemorrhages. The urothelium was mostly intact, although enzymes leaked out of the epithelial cells. The changes were increased at 24 h. The urothelium also showed some disruptions and degenerative vacuolization. The oedema reached its maximum at 48 h, and large numbers of inflammatory cells were also seen. The urothelium was disruptured in many places and vacuolated, but the subendothelial capillaries remained normal. Damage to some muscle cells was seen. After 7 days the oedema had disappeared and the urothelium was continuous. Enlarged nuclei were seen as white spots in the epithelial cells. Numbers of inflammatory cells were similar to those in the controls. Overdistension causes damage primarily to the bladder urothelium, and to a lesser extent to the muscular layer. Urothelium integrity is destroyed for several days, which makes it possible for various substances to penetrate the bladder wall and allows for bacterial adherence. The damage however is, almost completely healed within one week.