The cat and the rabbit are two of the most popular models for the study of lower urinary bladder function. The cat has been used extensively for in-vivo studies of spinal and supra-spinal micturition reflexes. In contrast, the rabbit has been used extensively for the in-vitro study of bladder function. In order to determine if the results obtained using one species can be applied to another, we have compared the in-vitro physiology and pharmacology of the cat and rabbit bladder using isolated strips and whole bladder preparations. The results can be summarized as follows: 1) The cat displays significant spontaneous activity during in-vitro cystometry, but the rabbit shows no such activity (whole bladder studies). 2) Although the bladder weights of the cat and rabbit are similar, the rabbit bladder has a capacity three times that of the cat. 3) The maximal response to field stimulation was obtained at one gram of passive tension for the rabbit isolated strips, whereas five grams of passive tension was required for the cat strips. 4) Atropine inhibited the response of isolated strips of cat bladder to field stimulation by approximately 13% whereas the response of rabbit bladder strips was inhibited by approximately 45%. 5) The magnitude of the response of rabbit bladder strips to ATP was similar to the response to field stimulation in the presence of atropine; the response of cat bladder strips to ATP was only 20% of the response of that of the rabbit bladder strips, and approximately 10% of the response of the cat strips to field stimulation in the presence of atropine. 6) Field stimulation produced a 10fold greater rise in intravesical pressure in the cat isolated bladder than in the isolated rabbit bladder; in response to bethanechol, the cat bladder generated a 6-fold greater response than the rabbit bladder. It is clear that the in-vitro pharmacological responses of the cat urinary bladder are qualitatively and quantitatively different from that of the rabbit bladder.