For the purpose of studies on vesicourethral reflexes, a model was established wherein the vesicosphincteric unit is divided into two separate compartments. In one group, the compartments were formed by single mucosal separation but muscular continuity was maintained. In the other, complete interruption of muscular continuity was performed by full bladder wall thickness separation. To maintain the proper nerve and blood supply to the upper compartment, the separation was made below the ureteral orifices and urinary diversion was accomplished through ureterosigmoidostomy. Postoperative cystographic studies demonstrated the complete separation. Detailed pre- and postoperative physiologic studies were done. Spontaneous contraction of the upper compartment excited contraction of the detrusor component of the lower compartment where muscular continuity was maintained. Contraction of the upper compartment induced reflex relaxation of the sphincteric element, regardless of whether the muscular continuity was maintained or interrupted. Flaxedil and Regitine both led to a drop in resting urethral pressure and abolished the drop in urethral pressure in response to contraction of the upper detrusor compartment. Although muscular contraction might be a contributory factor, it is clear that a nerve-mediated spinal reflex exists that induces drop in urethral resistance in response to detrusor contraction.