The cat and the rabbit have both been utilized extensively in the study of lower urinary tract function. Previous studies have demonstrated that although both the cat and rabbit bladder are approximately the same weight, the in-vitro cat bladder can generate over 6 times the intravesical pressure of the rabbit bladder. The current study was designed to compare the ability of the isolated bladder to generate pressure with the pressures required to maintain flow through the isolated urethra for both the cat and the rabbit. The results can be summarized as follows. 1) The cat bladder is visibly much thicker than the rabbit bladder, and in vitro cystometry demonstrates that it is far less compliant than the rabbit bladder. 2) Over 20 cm.H2O pressure is required to begin flow through the isolated cat urethra preparation, whereas 5 cm.H2O begins flow through the rabbit urethra. 3) Increasing the flow rate (up to 7-fold) through both the isolated cat and rabbit urethra increases intraurethral pressure only slightly. 4) Both the isolated cat and rabbit urethra respond strongly to field stimulation and alpha-adrenergic stimulation (relative to the opening pressure required to begin flow), but not to cholinergic stimulation. 5) Field stimulation following pre-stimulation by methoxamine induces a strong relaxation of the pre-stimulated cat urethra, but an additive contraction in the pre-stimulated rabbit urethra. These studies demonstrate that in order for the cat to empty its bladder, it must generate a comparatively high intravesical pressure, whereas the rabbit is required to generate a relatively low intravesical pressure.