Unilateral excision of the pelvic ganglion caused a loss in the number of AChE-positive nerves in the rat urinary bladder both on the operated side and on the contralateral side, thus indicating a bilateral intramural distribution of cholinergic nerves derived from the pelvic nerve. In the course of the subsequent observation period (3-28 days) the AChE-positive nerves increased in number and in staining intensity and further, the nerves became ramified and twisted. Similar events were found to occur in the urinary bladder decentralized on one side and denervated on the other. The morphological findings indicate an outgrowth of cholinergic nerves by collateral sprouting. These findings are discussed in relation to previous physiological studies.