The recent clinical success of distension therapy in the treatment of idiopathic detrusor hyperreflexia has prompted this ultrastructural study. The neuromuscular pathways of conduction in the rat and rabbit urinary bladder wall were examined using perfusion fixation techniques. The effects of short-term and prolonged distension were studied at frequent intervals for 4 months in treated and control animals. Prolonged distension did not alter either smooth muscle architecture or intercellular contact points. A transient phase of nerve degeneration in bladder muscle consisting of axonal swelling and lysis of organelles was observed. A quantitative estimate of nerve injury and regeneration was compiled using pooled histograms. Possible physiologic mechanisms of injury and clinical conclusions are discussed.