Bladder outlet obstruction has been the subject of numerous clinical and experimental investigations. Although these studies have demonstrated that the bladder can respond to outlet obstruction with muscular hypertrophy and hyperplasia, resulting in markedly altered morphological and functional characteristics, the nature of the direct effect of obstruction on the ability of the bladder to empty is relatively unknown. If one regards the bladder as a simple pump mechanism, decreasing the diameter of the outlet port (obstruction) would be expected to increase the work the bladder muscle would have to do in order to empty. The purpose of this present investigation is to better define the physical nature of the increased stress placed on the normal bladder via partial outlet obstruction. For these studies, the in vitro whole bladder is an appropriate model for several reasons: 1) one has complete control of initial intravesical volume, pressure, outlet diameter, and outlet resistance, and 2) this model quantitatively measures the effect of pharmacological agents and electrical stimulation on intravesical pressure and the ability of the bladder to empty. The results of these studies indicate that the obstructed bladder requires an increased pressure to empty. Although the obstructed bladder can empty completely, the rate of emptying is reduced significantly and the time required to completely empty is significantly increased. The obstructed bladder fatigues rapidly with repetitive stimulations, whereas the normal bladder is far less subject to fatigue. The physical alterations observed in these studies would play a direct role in the development of the functional alterations observed in in vivo obstruction.