The morphologic response of smooth muscle cells in the urinary bladder to a standardized bladder neck obstruction and the cell changes subsequent to release of that obstruction were studied in both growing and adult rabbits. The bladder reacted first by gross dilation and then by concentric thickening of the wall at the expense of the lumen. Microscopically, thickening of the bladder wall was due to temporary submucosal edema, then to the increased fibrocollagenous connective tissue, and finally to the increased muscle mass which in part replaced the connective tissue. Hypertrophy of the smooth muscle cells and their nuclei was apparent and this preceded cellular hyperplasia, which did not occur until 30 days after the obstruction was induced in the adult rabbits. In contrast, there was early hyperplasia of smooth muscle cells in the growing rabbits, followed by hypertrophy of the cells and their nuclei. In no instance could mitosis of smooth muscle cells be found in either young or adult rabbits. Quantitation showed that the degree of cellular hypertrophy was proportional to the duration of the obstruction. In the adult animal, the muscular thickening was mainly due to an initial 5 fold increase of the muscle cell volume together with a later 3 fold increase of muscle cell number. In the growing rabbits, the muscle cells first doubled in number and then after 8 weeks a 6 fold enlargement of cell volume contributed to the total muscular thickening. Upon release of obstruction in the adult rabbits, both enlarged cell and nuclear sizes regressed to normal values and the cell numbers gradually decreased to near normal values over periods up to 4 months after release.