The effect of prolonged bladder distension produced by elevating intravesial pressures under controlled conditions has been examined in rabits. Bladders were distended in vivo for 3 hr and studied at selected intervals during the postoperative period using light and electron microscopic techniques. In the acute phase, the continuity of the epithelium was partially destroyed and submucosal hemorrhages and edema were evident. However, these changes rapidly resolved and speciments obtained 7 days after distension were similar to controls. Using electron microscopy, smooth muscle cells were structrually unaffected by the experimental procedure and "junction" regions between adjacent cells were of normal appearance throughout the postoperative period. The arrangement and fine structure of autonomic nerves amongst detrusor smooth muscle were similar to those of control samples. These findings are discussed with reference to the findings of other workers and in relation to the use of bladder distension in the treatment of human detrusor instability.